My favourite (firefox) browser extensions:
Blocks annoying adverts, banners etc. A must-have if you want to browse without loading all the advertising junk. Some websites now detect adblock and refuse to let you on if it's installed. You can easily setup whitelists to stop adblock loading on certain sites. In my opinion, any site that detects and blocks adblock isn't worth browsing. (As of version 3.3 of Adblock Plus, it's nice to see Adblock taking a stand against the 'Anti-Adblock' websites). (sound of applause from over here)
Dark Background and Light Text
Makes every page have light text on dark background (exact colors are customisable). Instead of black, I use a dark grey for most pages. Makes websites much easier on the eye. Again, you can easily specify if a website uses it's own colours or the colours of this extension.
Kimetrak (Who's tracking me) is an extension allowing you see at a glance what third-party domains are loaded from websites you visit. You can access detailled statistics with a list allowing you to analyse what services are able to track you online.
Blocks cryptocurrency miners all over the web. It seems to work quite well, having stopped a good few in my experience. If you notice your CPU/GPU usage climb heavily (view in task manager) when browsing a website, it's worth installing to prevent your computer being used to mine Cryptocurrency without your permission.
Popup Blocker (strict)
Strictly block all popup requests from any website by default. A notification window is opened to allow you accept, reject, open the popup in background page, or redirect the current page to popup address.
Removes garbage (junk) like "utm_source" from URLs.
Remove anything (objects or selected text) from any page via the right-click menu. (useful for getting rid of things that might pop up on a website, designed to force you to do something).
Quickly and easily switch between popular user-agent strings. This is useful if you go to a website that says 'Your browser is too old/unsupported' - you can make the website believe you are running a different browser or operating system. Also useful to see how websites render differently with different web browsers.
Video DownloadHelper lets you download files from YouTube.com, Soundcloud.com and more. It's the only one that actually works at the moment for most videos!
It has a slightly jarring interface, and bear in mind you need the companion app installed to get the most out of it (for non-browser re-encoding).
Block popups, chat widgets, notification bars, and more from appearing on any website. Once an element has been blocked, this then becomes a saved rule stopping future page loads showing the matching item.
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