I use different browsers because, as Joe Offer says, they act 'differently'.

I recently changed my ISP from Comcast to Verizon, and now I can read posts on Facebook in only ONE browser...so far. It is Opera 11.11....which keeps asking me to update to 11.50 (and would do updates automatically if I checked that box) I HAVE a copy of 11.50, and can't read Facebook posts there. Why? Maybe some deep, hidden setting... *shrug* So far, I can't read Facebook in Firefox, either. Now, in ALL browsers I can get to my profile/settings page and it knows who I am and my ISP...it just won't show posts except in Opera 11.11.
See why I won't allow it to update? Other reasons-- I often find stuff by reading the browser cache, and Opera has totally changed the way the cache works after 10.10....making it much harder to find stuff--- so I have a copy of 9.64 using the old style when I want to, for example, find a song or an FLV (flash) file that is hard to get by direct download.
Firefox has also been using a more complex cache, but it makes it VERY hard to install different versions on the same machine (it seems to 'look' for other versions and demand to be installed in that folder).
   I have only played with Chrome a little bit, and currently my wife is using the only copy I have installed.
So far, I have avoided Safari, as it is based on the IE engine. (I also have Lunascape, which will run 3 engines, and allow YOU to choose which one) and K-Meleon and the last version of Netscape and 'SeaMonkey', which is a fancy version of Firefox)...the last 3 I seldom use.

Now.... with Firefox, you have thousands of 'add-ons' available which make it highly customizable...but too many of those and it loads VERY slowly. Opera has also started adding 'extensions' similar to Firefox, although they work differently.

Finally, the basic menu structure and settings in all browsers has changed over the years..(mostly to what I call dumbing down and allowing fewer options)...so I keep older versions for reference.
It seems security settings has driven many of the changes, and the desire to conform to business needs has driven many other changes..(like allowing Google to more easily track stuff)

Tedious? Too much? Overkill?... maybe. I do lose track at times of which one I have saved certain bookmarks in or which one has certain tabs open in. But...*shrug: some people have 4-5 bank accounts or 4 cars or 6 different Facebook profiles...( or several memberships in forums ☺)....or read 3-4 newspapers.

De gustibus non est disputandum