Beside the preinstalled Safari which I keep it updated, I also installed the other major web browsers: Firefox 5, Chrome latest beta (not the development one), and Opera the latest, currently version 11.50. There’s no Internet Explorer yet available for Mac, which I would install if only it existed 😀

I need to have those browsers to check how a website behaves in different browsers. Sometimes I find a website rendered differently in Chrome and Safari, although they both utilize Webkit, and there are those other browsers.

I use all those installed browsers quite extensively, but my primary browser now is Firefox, since version 5.

I choose Firefox as a primary browser for simple reason: I like the “Pin Tab” feature that unlike implementation on other browsers, it will stay pinned even if I close the tab. Then just clicking that closed pinned tab, will make it refresh its content, the original URL when it’s set to “Pin Tab”.
Other thing, now the close tabs on Firefox behaves just like Chrome: if I close a tab by clicking the small “x”, then the small “x” of the next tab will be placed under the cursor, no need to move cursor to find that small “x”. The tabs will be resized after we move cursor outside tab area. Convenient.

But, I find that the most stable browsers on my Mac are Opera and Safari. I never Force Quit those two. Firefox, Chrome, Chromium often need that.
And, Opera seems need the memory less. I launch Opera, open Gmail, and some other sites that I use often, and leave it on background.