I think Firefox is by far the best of the lot, and for my purposes (i.e. paranoia) it has a lot of useful add-ons for blocking scripts, cookies, ads, and other such crap I don't want to see and don't want on my machine. Slows things down a bit sometimes, but not enough to bother me, and for sites I trust, like this one, I can customize all the settings for what I will and won't allow.
With very few exceptions, I don't like the direction software developers are going, they all seem to be taking the lesson from Apple: you don't want to know what's going on, you don't need to know, don't ask, everything will be taken care of for you. Windows 7 I find won't let me control the machine to the extent I think I should be able to, I can't even own my own applications, some bozo called TrustedInstaller owns them. Internet Explorer and Outlook are security sieves, MS Office is bloated and stupid, Windows Defender is pretty much useless, the Norton and McAfee security suites have become hogs...ah, the list is endless. I use shareware and freeware as much as possible, Firefox for web browsing, Mozilla Thunderbird for email, OpenOffice for word processing and spreadsheets, and a variety of utilities like CCleaner, Defraggler, TweakNow, SiSoft Sandra, and some others. The only commercial software on my systems, apart from the OS, is Kaspersky's security suite.
Shouldn't be any hassle in switching, the Firefox installation routine knows IE is there and will ask you if you want to import various settings and favourites from it, I've never seen that go wrong. Firefox likely wouldn't speed up your computer generally, there are so many possible reasons for a system to be slow that installing one app isn't likely to make a difference. The obvious things to check first are disk fragmentation, for which I use a little thing called Defraggler (Defraggler - File and Disk Defragmentation - Free Download), and the number of processes and services you have running. The latter, however, requires some specialized expertise to know what's safe to shut down and/or uninstall. For that you really need someone in person to look over your shoulder and talk you through things, but as a quick guide, right click on the task bar at the bottom of the screen and run the Task Manager. On XP if it reports much more than about 30 processes running, and on Vista/Win7/Win8 if it shows more than around 50, chances are you've got things using up CPU and memory resources that you can do without. There's a guy calling himself Black Viper who's put together some pretty good and comprehensive guides to such matters. Black Viper's Operating System Guides
