Are we WebExtensions yet? Yes.

Mozilla documentation on WebExtensions.


§ Most Widely Adopted

Name Trackers Blocking bugs WebExtension
# 1-Click YouTube Video Download
YoutubeDownloader@PeterOlayev.com • 694409 users
# 1Password
onepassword4@agilebits.com
# ADB Helper
adbhelper@mozilla.org
# Adblock Plus
{d10d0bf8-f5b5-c8b4-a8b2-2b9879e08c5d} • 20619547 users
1226547
14 1190681
1210990
1213483
1269062
1277612
1305237
1310037
1364945
1366752
1369841
1370525
1370752
1371255
1388066
# Awesome Screenshot Plus - Capture, Annotate & More
jid0-GXjLLfbCoAx0LcltEdFrEkQdQPI@jetpack
1354137
0
WebExtension
# BetterPrivacy-signed
{d40f5e7b-d2cf-4856-b441-cc613eeffbe3}
# Download Flash and Video
{bee6eb20-01e0-ebd1-da83-080329fb9a3a} • 682182 users
# Download YouTube Videos as MP4
{b9bfaf1c-a63f-47cd-8b9a-29526ced9060}
# DownThemAll!
{DDC359D1-844A-42a7-9AA1-88A850A938A8} • 1294978 users
# Easy Screenshot
easyscreenshot@mozillaonline.com • 2403045 users
# Element Hiding Helper for Adblock Plus
elemhidehelper@adblockplus.org • 655424 users
# Flashblock
{3d7eb24f-2740-49df-8937-200b1cc08f8a} • 595268 users
# FlashGot Mass Downloader
{19503e42-ca3c-4c27-b1e2-9cdb2170ee34} • 848724 users
# Flash Video Downloader - YouTube HD Download [4K]
artur.dubovoy@gmail.com • 1066245 users
# Ghostery
firefox@ghostery.com • 1464992 users
WebExtension
# Google Translator for Firefox
translator@zoli.bod • 798165 users
# Greasemonkey
{e4a8a97b-f2ed-450b-b12d-ee082ba24781} • 1302204 users
1353468 1334174
2 1332273
1353468
# IDM CC
mozilla_cc@internetdownloadmanager.com
# LastPass Password Manager
support@lastpass.com • 745753 users
1282579
1 1282972
# Lightbeam for Firefox
jid1-F9UJ2thwoAm5gQ@jetpack • 509680 users
1342681
1 1345158
# MEGA extension
firefox@mega.co.nz
1219940
1 1246236
# NoScript Security Suite
{73a6fe31-595d-460b-a920-fcc0f8843232} • 2146154 users
1214733
5 1210990
1213483
1255894
1305237
1332273
# Pin It Button
jid1-YcMV6ngYmQRA2w@jetpack
# Reddit Enhancement Suite
jid1-xUfzOsOFlzSOXg@jetpack
1208765
0
WebExtension
# Tab Mix Plus
{dc572301-7619-498c-a57d-39143191b318} • 842022 users
1226546
0
# uBlock Origin
uBlock0@raymondhill.net • 1154406 users
12713541309926
2 1313401
1367259
WebExtension
# Ubuntu Modifications
ubufox@ubuntu.com
# Valence
fxdevtools-adapters@mozilla.org
# Video DownloadHelper
{b9db16a4-6edc-47ec-a1f4-b86292ed211d} • 4452660 users
1310316
4 1246236
1255894
1271553
1323414
# Web of Trust, MyWOT/WOT: Website Reputation Rating
{a0d7ccb3-214d-498b-b4aa-0e8fda9a7bf7}
# Xmarks Sync
foxmarks@kei.com • 337623 users
# Yahoo Toolbar and New Tab
{635abd67-4fe9-1b23-4f01-e679fa7484c1} • 716458 users
# YouTube䋢 Video and Audio Downloader
feca4b87-3be4-43da-a1b1-137c24220968@jetpack

§ Popular

Name Trackers Blocking bugs WebExtension
# Grammarly for Firefox
87677a2c52b84ad3a151a4a72f5bd3c4@jetpack
WebExtension
# AngularJS Batarang
ighdmehidhipcmcojjgiloacoafjmpfk@chrome-store-foxified-1964081006
# Ember Inspector
ember-inspector@emberjs.com
# React Developer Tools
@react-devtools
WebExtension
# Redux DevTools
extension@redux.devtools
WebExtension
# Vue.js devtools
nhdogjmejiglipccpnnnanhbledajbpd@chrome-store-foxified-1617803097
# HTTPS Everywhere
https-everywhere@eff.org
1390116
0
# Norton Identity Safe
idsafe@norton.com
WebExtension
# Norton Safe Web
nortonsafeweb@symantec.com
WebExtension
# Adobe Acrobat - Create PDF
web2pdfextension@web2pdf.adobedotcom
# AdBlocker for YouTube™
jid1-q4sG8pYhq8KGHs@jetpack • 314420 users
WebExtension
# AdBlocker Ultimate
adblockultimate@adblockultimate.net • 357692 users
WebExtension
# Adguard AdBlocker
adguardadblocker@adguard.com
# anonymoX
client@anonymox.net • 549250 users
1371270
0
# Ant Video Downloader
anttoolbar@ant.com • 407227 users
# Avast Online Security
wrc@avast.com
# AVG Web TuneUp
avg@toolbar
# Avira Browser Safety
abs@avira.com
# Blur
donottrackplus@abine.com
WebExtension
# browser-security-1
firefox@browser-security.de
# Browsing Protection by F-Secure
ols@f-secure.com
# ColorfulTabs
{0545b830-f0aa-4d7e-8820-50a4629a56fe} • 250595 users
# ColorZilla
{6AC85730-7D0F-4de0-B3FA-21142DD85326} • 273013 users
WebExtension
# Dashlane
jetpack-extension@dashlane.com
# Disconnect
2.0@disconnect.me • 283780 users
WebExtension
# Download Statusbar
{D4DD63FA-01E4-46a7-B6B1-EDAB7D6AD389} • 331909 users
# Easy Youtube Video Downloader Express
{b9acf540-acba-11e1-8ccb-001fd0e08bd4} • 326119 users
WebExtension
# EPUBReader
{5384767E-00D9-40E9-B72F-9CC39D655D6F} • 424322 users
1341098
2 1341341
1356397
WebExtension
# Evernote Web Clipper
{E0B8C461-F8FB-49b4-8373-FE32E9252800} • 391479 users
WebExtension
# Firefox All Aboard 1.6
@all-aboard-v1-6
# FireGestures
firegestures@xuldev.org • 363752 users
# Flagfox
{1018e4d6-728f-4b20-ad56-37578a4de76b} • 525145 users
# FoxyProxy Standard
foxyproxy@eric.h.jung • 286381 users
1359543
1 1360404
# Full Web Page Screenshots (♥♥♥♥♥)
{0b457cAA-602d-484a-8fe7-c1d894a011ba} • 279529 users
# Garmin Communicator
{195A3098-0BD5-4e90-AE22-BA1C540AFD1E} • 418313 users
# Google Translate, ImTranslator, Dictionary, TTS
{9AA46F4F-4DC7-4c06-97AF-5035170634FE} • 336652 users
# iCloud Bookmarks
firefoxdav@icloud.com • 383932 users
1293853
0
# IE Tab V2 (Enhanced IE Tab)
{1BC9BA34-1EED-42ca-A505-6D2F1A935BBB} • 515934 users
# Image Zoom
{1A2D0EC4-75F5-4c91-89C4-3656F6E44B68}
# Kaspersky Protection
light_plugin_F6F079488B53499DB99380A7E11A93F6@kaspersky.com
# McAfee Security Scan Plus detection
{e4f94d1e-2f53-401e-8885-681602c0ddd8}
# McAfee WebAdvisor
{4ED1F68A-5463-4931-9384-8FFF5ED91D92}
# MeasureIt
{75CEEE46-9B64-46f8-94BF-54012DE155F0}
# Mozilla Archive Format, with MHT and Faithful Save
{7f57cf46-4467-4c2d-adfa-0cba7c507e54}
1332920
0
# NetVideoHunter
netvideohunter@netvideohunter.com • 278207 users
# New Tab Homepage
{66E978CD-981F-47DF-AC42-E3CF417C1467}
# NoSquint
nosquint@urandom.ca
# Password Exporter
{B17C1C5A-04B1-11DB-9804-B622A1EF5492}
# Personas Plus
personas@christopher.beard • 312913 users
1369209
0
# ProxTube - Unblock YouTube
ich@maltegoetz.de • 311056 users
# QuickJava
{E6C1199F-E687-42da-8C24-E7770CC3AE66} • 274114 users
# ReloadEvery
{888d99e7-e8b5-46a3-851e-1ec45da1e644}
# S3.Google Translator
s3google@translator • 369330 users
# SaveFrom.net helper all-in-1 / youtube downloader
helper@savefrom.net
WebExtension
# ScrapBook
{53A03D43-5363-4669-8190-99061B2DEBA5}
# Self-Destructing Cookies
jid0-9XfBwUWnvPx4wWsfBWMCm4Jj69E@jetpack
# Skype
{82AF8DCA-6DE9-405D-BD5E-43525BDAD38A}
# Speed Dial
{64161300-e22b-11db-8314-0800200c9a66} • 378805 users
# Speed Dial [FVD] - New Tab Page, Sync...
pavel.sherbakov@gmail.com
# Status-4-Evar
status4evar@caligonstudios.com
# Stylish
{46551EC9-40F0-4e47-8E18-8E5CF550CFB8} • 481414 users
# Text Link
{54BB9F3F-07E5-486c-9B39-C7398B99391C}
# User Agent Switcher
{e968fc70-8f95-4ab9-9e79-304de2a71ee1} • 343383 users
# Video Downloader professional
ffext_basicvideoext@startpage24 • 258678 users
1283654
1 1282982
WebExtension
# Web Developer
{c45c406e-ab73-11d8-be73-000a95be3b12} • 493583 users
# xThunder
xthunder@lshai.com
# YouTube ALL HTML5
jid1-qj0w91o64N7Eeg@jetpack
# Youtube Downloader 4K - Video Downloader
paulsaintuzb@gmail.com
# YouTube Flash Player
jid1-HAV2inXAnQPIeA@jetpack • 402546 users
WebExtension
# YouTube Flash Video Player
{f3bd3dd2-2888-44c5-91a2-2caeb33fb898} • 299809 users
# YouTube High Definition
{7b1bf0b6-a1b9-42b0-b75d-252036438bdc} • 284115 users
# YouTube mp3
info@youtube-mp3.org • 514302 users
# ZenMate Security & Privacy VPN
firefox@zenmate.com • 285639 users
# Classic Theme Restorer
ClassicThemeRestorer@ArisT2Noia4dev • 436611 users
# frigate
e67f8350-7edf-11e3-baa7-0800200c9a66@fri-gate.org • 258320 users
# Amazon Assistant for Firefox
abb@amazon.com • 280014 users
WebExtension
# VKontakte.ru Downloader (music/videos/photos @ VK)
vk@sergeykolosov.mp
# Session Manager
{1280606b-2510-4fe0-97ef-9b5a22eafe30}
# Easy Access
quicklaunch@mozillaonline.com • 303194 users
# Yahoo Toolbar and New Tab
{635abd67-4fe9-1b23-4f01-e679fa7484c1} • 716458 users
# Search and New Tab by Yahoo
jid1-16aeif9OQIRKxA@jetpack • 582449 users
# New Tab by Yahoo
jid1-G80Ec8LLEbK5fQ@jetpack • 318370 users
# FireFTP
{a7c6cf7f-112c-4500-a7ea-39801a327e5f}

§ Other

Name Trackers Blocking bugs WebExtension
# Session Manager
{1280606b-2510-4fe0-97ef-9b5a22eafe30} • 288482 users
# VKontakte.ru Downloader (music/videos/photos @ VK)
vk@sergeykolosov.mp • 275379 users
# Mozilla Firefox hotfix
firefox-hotfix@mozilla.org • 283311 users
# Yandex.Bar
yasearch@yandex.ru • 452947 users
Source on github.