Mozilla Firefox
In Windows, Firefox is my main browser for about a year now, previously I use Opera mostly.
Don’t get me wrong, it’s not because Opera is inferior to other browsers…
My top reason I use Firefox, because I use it all the time in Ubuntu Linux, so rather than use different browser in Windows, I switched to Firefox.
Then, there are some plugins that I love, though I can use other softwares, but it is nicer if it is ready all the time in your always-open browser 
I use many plugins, but I have some plugins that always to be plugged: GMail Manager, GMail Filespace, FireFTP, Bookmark Duplicate Detector, OpenBook, Foxmarks Bookmark Synchronizer, IE Tab, KeyConfig, and Tab Mix Plus.
And oh ! Aardvark is also very very helpful to clean up site before saving or printing, it removes unneeded objects.
…and that IE Tab, you will also find it very useful. But why using IE in Firefox ? There many answers for that, from testing a site that you just developed, until because of the laziness of a webmaster that make you have to browse the site using IE …
And there are DownThemAll and FlashGot. Both are useful, and FlashGot also support many download managers including Free Download Manager.
And the last plugin make your task easier with Firefox is All-In-One Sidebar, that able to host all sidebar in it, just as it said.
I think I’ll post my review of plugins I use and put link of each plugins, but for time being you can always find all those mentioned plugins at Mozilla Firefox site.
