Be faster than Chrom(ium) with regards to start up speed and JS execution.
I abandoned FF after having used it since it was "Phoenix 0.1", because it had become slow and bloated, like NN had been before. Would love to switch back, but not if I can open 5 Chromium instances in the same time it takes to open one FF window.
Have you tried a new profile? A long-time user will end up with a really crufty Firefox profile. Sync will move all your data to a new profile quickly and you can see what a new profile in Firefox is like.
Sync will not move all your data. I switched to a new profile after installing an add-on that introduced problems and uninstalling didn't correct those problems. I fell back to Sync for the new profile so I could salvage my history and bookmarks from my 2-ish year old profile, but it left out something like 9 months of history, all but two of my tags, and a good chunk of my bookmarks and search keywords (including the ones I'd set up for MXR).
I'm not sure if it was buggy, or if Sync was intentionally not tracking the stuff that didn't get copied over. (cf mconnor's blog post a month or so ago about implementing Sync quota caps).
I don't use Firefox at all, but I just did some testing and the first time I started Firefox it took about 3 seconds to show its window. Subsequent starts got faster but always took at least a second and half. Chrome starts before I can say "one".
I abandoned FF after having used it since it was "Phoenix 0.1", because it had become slow and bloated, like NN had been before. Would love to switch back, but not if I can open 5 Chromium instances in the same time it takes to open one FF window.