That's a very arbitrary line to draw. Some "alternative browsers" exist that wrap the IE rendering engine in a different UI (and some of those include their own firebug-like functionality), does that mean IE is more customizable than firefox (which, after all, forces you to use GTK for your UI)?
Wrapping the IE rendering engine in a different UI is a different browser, but further than that, you can also embed Gekko, Firefox's rendering engine in a custom UI.
Examples of browsers that embedded Gekko: Camino (a Firefox fork for OS X that happened in a time when Firefox wasn't as polished for OS X), Flock and K-Meleon. Google's Picassa for Linux was also using Gekko.
Only Firefox on Linux uses GTK. Even on Linux, there have been previous attempts at supporting Qt as the backend for KDE, but all failed because of the easiness with which you can make GTK look like whatever KDE theme you've got selected - not perfect, but the flaws where not enough to gather interest in further development.
This isn't to say that XUL/Gekko are perfect as their complexity was often the subject of criticism, which is why Mozilla replaced XUL completely with HTML5 in Firefox OS and will probably do so in future Firefox versions - as they are also working on Servo, a next-gen rendering engine that doesn't do XUL anymore: http://www.mozilla.org/en-US/research/projects/
Imagine a browser who's every facet and functionality is customizable by HTML5/Javascript extensions that you can install with one click. That's what Firefox already is - the Emacs of browsers.
For example, if my Firebug example wasn't enough, when Chrome was released, many people loved the light download progress functionality that wasn't opening an annoying modal window. Pretty soon an extension called the Download Statusbar happened: https://addons.mozilla.org/en-US/firefox/addon/download-stat...
>Wrapping the IE rendering engine in a different UI is a different browser, but further than that, you can also embed Gekko, Firefox's rendering engine in a custom UI.
>Examples of browsers that embedded Gekko: Camino (a Firefox fork for OS X that happened in a time when Firefox wasn't as polished for OS X), Flock and K-Meleon. Google's Picassa for Linux was also using Gekko.
True, but only goes to show my point - if firefox's extension mechanism made it so super-customizable, surely there would be no need for such browsers?
>Firefox's UI toolkit is not GTK, but rather XUL+XPCOM
Fair enough, but the point stands; when writing extensions you're restricted to using the XUL toolkit. Contrast with e.g. activex-based add-ons in Internet Explorer, where AIUI you get the standard windows API and can thus use any toolkit you like.
>Imagine a browser who's every facet and functionality is customizable by HTML5/Javascript extensions that you can install with one click.
I'm happy to believe that Firefox is the browser that's easiest to customize in HTML5/Javascript, I just think that's a very arbitrary line to draw. IE addons can be any language you like (because again they're just using the standard APIs) and can be installed with one click.
There are plenty of good things about firefox, but I don't think you can say it's more or less customizable than the alternatives without defining customizability in a very arbitrary way. All browsers have a succession of methods of customization, from simple userjs to custom extension formats to embedding the engine in a new executable, with the power and complexity increasing at each step. That firefox's "extensions" lie at a bit more powerful and complex point along the line than chrome's is not the basis for this blanket claim of greater customizability.
Mozilla is working on it. Firefox OS for example no longer uses XUL and they've got an awesome research project called Servo - a Gekko replacement that addresses its short-comings.
Name another browser where the Firebug-like functionality is an extension and not something built-in.