Why I have so many browsers

Do I really need more than one browser to use?. Actually, I use Mozilla Firefox for my main browser, but since I have 2 gmail account, I need to open it in separate browser (so I don't have to logout and login for each account). The second browser is also from Mozilla, that is SeaMonkey. I use seamonkey because I need the composer and to pop gmail. I used to pop my mail from Thunderbird, but I think now Seamonkey is enough for me.

The third browser is Opera. I'm using it to open some site that need other google acount too. (for my boyfriend). I almost forget, in the windows there already Internet Explorer, I never use that until my internet banking page only can open in IE. I already ask my bank to improve their program so I don't need IE in the future.

I like mozila browsers, and not feel safe when using IE :(. Anyway, mozilla browsers available for my windows and fedora platform.