There are two bright points to using Firefox: 1) Firebug and 2) it isn't Chrome.
The first one is a personal preference - Inspector seems weird to me and while it may have most of the needed functionality, it doesn't feel like it does.
The second is because there isn't any way I know of to have Chrome start in clean mode (with no bookmarks, etc. from personal use). So, I test and debug in FF and only use Chrome for news, apps, etc.
When it works, Firebug is good and useful, though with some Firefox "Nightly" versions, Firebug is broken. Of course, I know, that's what I get for running Nightly...
The Firefox developer tools have matured and recent iterations work a lot like Firebug so I miss it less.
Your second point is very much on target. Because of problems rendering web pages, I haven't found Chrome and webkit-based browsers useable for developing or testing web-based interfaces.
Pages that look good and essentially the same in Firefox and IE will have strangely different fonts (and sometimes layout) when viewed in Chrome, Opera, Safari, etc. Users of these browsers sometimes notice my sites appear a little misshapen and ask what to do. I suggest trying Firefox, or in a pinch, IE.
This is less an issue with mobile viewing since page rendering is different on those devices vs. desktop/laptop anyway. Of course, browser development travels at near light-speed, so the situation can change quickly. But given the mix of old and new systems out there, it's a headache that won't go away for quite a while.
The first one is a personal preference - Inspector seems weird to me and while it may have most of the needed functionality, it doesn't feel like it does.
The second is because there isn't any way I know of to have Chrome start in clean mode (with no bookmarks, etc. from personal use). So, I test and debug in FF and only use Chrome for news, apps, etc.