My favourite (firefox) browser extensions:

 Adblock Plus
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)

 Anti-Adblock Blocker
After I install Adblock, I always install this too. Some websites try to prevent you from using Adblock. This extension blocks their attempts to prevent Adblock from working. In the never-ending war against adverts and content that I haven't asked for, this is a must. It helps stem the flow of the preverbial 'vomit of adverts' which have found their way onto every page on the internet.

 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
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.

 CSS Override
CSS Override alllows you to set your own custom CSS in a web page. Once the web page loads, you can set additional CSS rules to permanently customise, hide, or otherwise change the appearance of annoying elements of any website.

 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.

 Pure URL
Removes garbage (junk) like "utm_source" from URLs.

 Remove Anything
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).

 User-Agent Switcher
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.

 GMail Notifier
Plays an audio alert for your Gmail notifications. Doesn't store your password, doesn't need a restart, and is customisable.

 Element Blocker
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.

 I don't care about cookies
Gets rid of those (endless) alerts regarding cookies on most websites if you are sick and tired of seeing them.

 Force Paster
Allows copy and pasting of text into any website, even if the website tried to block it.

 

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