Add-ons

Recommended browser and email extensions. Descriptions are from the extensions themselves.

First: Not recommended (all platforms)

• AdBlock, Adblock Plus, Ghostery, Disconnect (potential conflicts of interest)
• Coupon/deal tools (Honey, Microsoft Rewards, etc)
• Quasi-security add-ons (Securify, SecuryBrowse, etc.)
• "Helper" apps (Media New Tab, wiBit, Email Helper App, TVPlusNewTab, Showseeker, etc.)
• Third-party PDF tools (YouPDF, ViewPDF, etc) - use Adobe or browser built-in PDF tools only

Chrome / Chromium / Edge

Security and privacy - core, all users

• Google Analytics Opt-Out (by Google)
• HTTPS Everywhere (by EFF) - force all optional HTTPS pages to require it - (see also Edge version)
• IBA (Interest-Based Ads) Opt-out (by Google)
• Privacy Badger (by EFF) - reduce tracking footprint, from an independent non-profit (not an ad company or a company that owns ad companies) - (see also Edge version)
• uBlock Origin (by gorhill) - best ad blocker by an order of magnitude
• Windows Defender Browser Protection (Microsoft)

Security and privacy - core, advanced users

• ScriptSafe (for intermediate/advanced users)
• Suspicious Site Reporter (by Google)

Security and privacy - other

• FlashBlock - FlashBlock for Chrome. Block them all, or be selective with the embedded allowlist manager (some functionality now default in Chrome)
• HTTP/2 and SPDY indicator
• SecurityHeaders.io Analyser - check security headers for the current site
• Ugly HTTP - make HTTP-only sites visibly different using configurable themes
• Wizmage Image Blocker - Only see the images you want to see, on a site-by-site bases (allowlist or denylist)

Web development

• Check My Links - check all links on page for existence/redirect, with fast visual indicator of status
• moarTLS Analyzer - check all links on page for HTTPS - with fast visual indicator of status
• Show Chrome Version - Simple major/minor version display in toolbar, with fresh/stale status - by @ericlaw
• Stylish - Restyle the web with Stylish, a user styles manager
• Web Developer - Adds a toolbar button with various web developer tools

Other

• Animation Policy (by Google) - run animations once, or disable them completely
• Personal Blocklist (by Google) was fantastic, but has been removed from the Google Play store
• Search by Image has been replaced by Google Lens, but you can restore the old behavior
• Share Extensions - Export your favorite Google Chrome extensions as BBCode, HTML, Wiki or text
• Social Fixer for Facebook - Make Facebook usable again [recommendation: only run Facebook on desktop in Firefox using the Facebook Container extension]
• Video Speed Controller by Ilya Grigorik of Google - simple, effective speed control for any HTML5 video, with hotkeys

Firefox

Security and privacy

• Certainly Something - thorough cert viewer by April King
• CookieSafe Control cookie permissions.
• HTTPS Everywhere (by EFF) - Encrypt the Web! Automatically use HTTPS on many sites.
• Laboratory (CSP Toolkit) - generate a CSP header for your website
• NoScript - NoScript allows JavaScript, Java (and other plugins) only for trusted domains of your choice
• Privacy Badger (EFF)
• uBlock Origin - ad blocker (by gorhill)
• uMatrix - Javascript control framework (by gorhill)
• Facebook Container (by Mozilla) - isolates Facebook in its own browser sandbox(ish)
• Social Fixer - make Facebook work like it should

Web development

• Html Validator Adds HTML validation to the View Page Source of the browser. The validation is done by Tidy from W3c.
• RESTClient
• User Agent Switcher and Manager
• Wappalyzer
• Web Developer Adds a menu and a toolbar with various web developer tools.

Other

• DownThemAll! The mass downloader for Firefox.
• FoxClocks FoxClocks shows times around the world - or just your local time - at the bottom of your browser
• Greasemonkey A User Script Manager for Firefox
• LinkVisitor Mark link(s) as visited or unvisited
• SQLite Manager Manage any SQLite database on your computer
• Table2Clipboard2 Allow to copy to clipboard the selected rows/columns from an HTML table
• Update Scanner Monitors webpages for updates

Thunderbird

• Auto Copy 2 - Copies selected text to the clipboard automatically. Like Linux or mIrc
• ConfirmFolderMove - Introduces a preference that allows users to be prompted before moving folders
• Correct Identity - Allows you to configure the identity that will be chosen upon composing mail
• Folderpane Tools - Allows for customization of the folder pane such as rearranging accounts and choosing a startup folder
• LookOut
• Mail Redirect - Allow to redirect (a.k.a. "bounce") mail messages to other recipients
• Mnenhy - Mozilla enhancements with special focus upon Mail&News
• NestedQuote Remover - Remove nested quotes when replying to a mail
• NotTo - specify who not to send to - excludes from an existing group
• Provider for Google Calendar - Allows bidirectional access to Google Calendar
• Quickfile - Quick keyboard based message filing and navigation extension
• Quicktext - Adds a toolbar with unlimited number of text to quickly insert. It's also possible to use varibles like [[TO=firstname]]. With settings for everything.
• QuoteCollapse - Collapse quotes.
• Remove Duplicate Messages - Removes duplicate messages by folder context menu.
• Right Click Watch Ignore - Adds "Watch Thread" and "Ignore Thread" to right click context menu in Thunderbird
• Signature Switch - Switch the signature on/off or choose a new one from your predefined set.
• SmtpSelect - Toolbar button for selecting the default SMTP server.
• Toggle Word Wrap - Toggle word wrapping in Thunderbird (Composer window) or Firefox (<PRE> elements)