Got fed up with FF when it became virtually unresponsive on a Vista system I was the lucky recipient of. Tried Chrome but never saw the speed from it amost comparisons claim. Found Safari to be reasonably quick even on Vista, and I liked the interface, but the controls for enabling and disabling javascript and such leave a little to be desired. After switching to Windows 7 I started having problems with Safari bombing out for no apparent reason. Which is how I ended up at Opera. After 18 months it is still my favorite. Though I wish I knew Norwegian as it would make finding add on’s easier. 
Firefox works very well indeed on Linux systems. I dual-boot, but I find I’m only using Windows for non-internet tasks; all the online stuff works better with Linux.
I know that AdBlock is available on other browsers now, but I feel no need to change. AdBlock, to me, is essential.
Firefox 4 is much, much faster for me on multiple Windows computers.
I’ve switched back and forth between Chrome and Firefox a few times, though now I am primarily using Firefox 4.whatever. Chrome is definitely faster at rendering a page, but it doesn’t let you navigate around or see much of anything until the page is completely rendered. Firefox is slower, but you can see a partly rendered page, so you can start reading some of the text while the renderer catches up. I’ve mostly switched to Firefox because I like a few of its features better, particularly the awesome bar and dedicated search box.
But I’m definitely feeling the bloat. I understand that having several dozen tabs open at once will take a lot of memory and eventually some of the pagefile. Even after closing those tabs, though, Firefox still hogs a big chunk of memory, and isn’t responsive until I restart it.
The Mozilla team is aware of the problem and they’ve recently started a project to fix the memory issues.
I am using FF 3.6, and have not noticed any slowing problems at all (Windows XP). I tried ver 4 and hated it, so went back.
However, am intrigued with info about SeaMonkey, so will try it. Just saying the name and that it is my browser will be fun.
I just downloaded Seamonkey and gave it a try. An hour later I got rid of it. The very first thing I needed was to move the tabs to the bottom of the screen (below the content, like Opera has it). Can’t be done. Also I could not add the “new tab” button to the menu bar. Oh well, an hour wasted.
They know about the fullscreen issue in FF4 (to give just an example) ever since they started it. Two updates later, the issue is still there. POIDH…
What an odd request… I’ve never heard of that layout before. But it seems you’re not the only one who wants it.
There’s currently no user-friendly way of setting the position of the tab bar, but if you don’t mind editing text files, then just add this line to the userChrome.css file in your profile directory and restart the browser:
#content > tabbox {-moz-box-direction: reverse;}