Not to discredit Firefox for spearheading our revolt against the IE monopoly, but I think Firefox has sometimes been given too much credit for everything from standard compliance to innovation, and not enough credit to other browsers on the market.

First of all, standard compliance. I don't think Firefox 1.0 or its ancestors and subsequent version are all that standard compliant. Yes, for about 1 year in 2004-2005 it was the most standard compliant browser on the planet and could display most sites not written specifically for IE the way they were meant to be. But that drive to remain standard compliant quickly evaporated. When Safari 2.0 came out in 2005, it passed Acid2 and continued to be the most standard compliant, fastest browser on the planet with the cleanest code base for the next couple of years. And yes, it could render sites just as well as Firefox. Firefox OTOH, didn't pass it until 3.0 came out in 2008, and the second to last to pass Acid3.

Secondly, usability. I don't think Firefox from the day it came out to the present day is all that great in terms of UX. People seem to have forgotten that most of the browser UI features we take for granted came from Opera and later Chrome. Tabbed browsing? Opera first. Tab preview? Opera first. Most visited sites? Opera first. Tab groups? Opera first. Download Manager that speaks BT? Opera first. Mouse gestures? Opera first. The driver to innovative browser UI had always been Opera ever since IE dominated the market until Chrome came along in 2008 with its tab in title bar and omnibox. The only thing Firefox pioneered is the 3D view in the newer Firefox's inspect bar, which they somehow prioritized over consolidating all those overlapping and confusing developer tools, and making live editing in the markup panel possible, as in Firebug had been doing since 2007ish.

Third, under the hood innovation. Since the Netscape era to the present day I think the one most prevalent problem is misplaced focus. Mozilla simply has way too many sub-projects going on, not all of them important, and all of them compete for resources. Just look at how many man-hours Mozilla devs have wasted on technology that no one likes to use / can use but nevertheless bloat up Gecko. XPCOM, XUL, all those W3C alphabet soup X* standards in the early days, and then later on putting more and more JS features like let and generators into the JS engine. The APIs used to write extensions were a TERRIBLE mess before FUEL came out.

Lastly, speed, memory usage, and stability. Not until last year Firefox' launch speed was the slowest of all. It was unbearable and had not been addressed until very recently. Same for memory usages. In terms of JS exec speed and rendering speed, it was also among the slowest, only slightly faster than IE, though there has been significant improvement in this area as well this year. In terms of stability, it was not until recently that it got a boost using the same technique Chrome pioneered.

I can keep going on and on but the one thing that I cannot stress more is Mozilla's inability to focus on what's important. There was a time when Netscape had 80% of the market and Firefox a trend to break through 40% usage, but innovation stagnated while Safari, Chrome and Opera have continued to improve by leaps and bounds. There seems to have been many significant improvements to these problems recently and I certainly hope to see more aggressive innovation and focus from the open source community around Mozilla.