Mozilla Firefox
Version 20.0 (Ubuntu 12.04.2 LTS)
Nearly everyone has heard of Firefox (FF). Not quite everyone because Mozilla doesn't pay advertising for it, but it has such a strong community that word of it gets around easily. Mozilla is undoubtedly the most moral company out there. It makes no profit, yet (thanks to much generosity from companies like Google, Canonicle (the company behind Ubuntu), Red Hat (the company behind Fedora) and, oddly, Oracle) it is very rich and can have quite a few paid developers for its projects like Firefox without having to have its customers pay anything and there is no need for annoying advertisments.
It doesn't only have a strong community and a moral company, it is the lightest open-source browser out there. Internet Explorer (IE) is reputed for being slow (although IE10 Modern (Windows 8 or Metro) is EXTREMELY lightweight and is THE fastest browser out there but it only runs on Win8, is touch-based and is closed-source), Chrome isn't completely open-source (Chromium is but it is much slower and a little incompatible with Chrome) and is getting more bloated with each update, Opera is totally proprietary (closed-source) and very unpopular (although Opera is the only main up-to-date browser to support Windows 2000) and Safari doesn't come close to the speed of the others and is owned by the tyrant company Apple.
Firefox is very fast on low-end system compared to other popular browsers and it still has many features including: Personas (you can customise the whole appearance of Firefox with these, I have a massive Minecraft landscape on mine, this doesn't look as good on Chrome), App Tabs (very tiny tabs that always load when you start FF but they don't slow the main page down, they also light up when something updates on them, very useful for keeping an eye on your Twitter/Facebook/Google+ feed), the Awesome Bar (works in the same way as the Multibar and IE9/10's Address Bar but you can scroll down your results while typing) and more...
You can also add more features with a very detailed add-on page. For example, I have Flash Video Downloader, a very useful Empty Cache Button and a Download Status Bar (so I get a bar which is better than the Chrome download circle). Some may say that Chrome gets web apps and FF doesn't...that isn't true, Chrome apps are coded in HTML5 and if you know where to look, FF can load them too, probably better.
In conclusion, if you are on Windows Vista/7/8/RT, you should consider using it for its awesomeness and open-sourceyness, if you are on any other operating system (including Windows XP), Firefox is a necessity.
Usability: 9/10 (Only one defect and that is that there is no Duplicate Tab button, any other 'defects' are solved with add-ons)
Aesthetics: 9/10 (The default depends on your operating system, but the Personas are fantastic)
Performance: 9/10 (One of the fastest browsers on the planet, supposedly only surpassed by IE9/10 and Chrome (although on Ubuntu, FF appears to be even faster than Google Chrome)
Overall: 9/10 (No browser is perfect, this is probably the best one and none will surpass it because of the massive collaboration work on FF)
Written by Adam.