Interesting Firebug story: Years ago when Netscape/Firefox was being built, some developers wanted to put a bunch of developer tools into the browser. Mostly so Firefox developers could inspect the browser (remember, websites were pretty bland back then).
However, they were worried about the codebase becoming too big -- after all, 99.9% of people wouldn't need a DOM inspector. So, the add-ons architecture was born. Firebug was created as a separate tool by Joe Hewitt. Add-ons and Firebug probably ended up being the reason why Firefox took off.
One of the reasons. I think tabs were actually a bigger deal back then. Although Opera had them first, Firefox was the one that put them front and center. Back then, Opera was still championing a gesture-based interface, and Firefox felt a lot more solid and polished.
Opera had little miniature windows inside the main window. A small difference, perhaps, but tabs are simpler to manipulate at the cost of, perhaps, being less flexible (you can't resize tabs and move them around like you can windows).
Actually this is incorrect -- the windows you're referring to were part of the earlier MDI version of Opera. In 2000 Opera 4 came out with tabs very similar to what FireFox would have in its first release four years later (and its predecessor Phoenix had in 2002).
I agree with keypusher.
Firefox 'took off' long before the whole add-ons/extensions became popular. It was being able to use tabs which caused many too switch back in the day (plus it was faster to, or at least felt that way). And yes I'm also aware opera did this first.
actually firefox took off because it wasn't IE :) remember that those were the days that you either used IE on windows or nothing since netscape was dead and there weren't too many choices out there for a free browser that worked well.
However, they were worried about the codebase becoming too big -- after all, 99.9% of people wouldn't need a DOM inspector. So, the add-ons architecture was born. Firebug was created as a separate tool by Joe Hewitt. Add-ons and Firebug probably ended up being the reason why Firefox took off.