That's insane -- I thought I was the only one who used multiple browsers!
I do my work stuff on Chrome, writing, blogging, and OC.net stuff on Firefox, non-English language content on Opera, and IE...well, I access bookmarks from several years ago.
When people use my laptop, I give them FULL instructions (I usually let them use Chrome, since people keep infecting my computer when they use the IE browser) and refuse to let them fool around on my Firefox, since I usually have multiple windows open, and they "accidentally" close all of them.
It's less of a tech geek thing than an organization thing.
I'm on the internet for hours and hours on end, and I can't even count how many pages I visit a day for work, pleasure, research, etc. Putting it all in one browser would be hectic!