Back in the Mozilla days i was using Mozilla because it was very powerful and flexible despite being very slow (i remember watching Mozilla 0.6's dialog boxes draw themselves) and many pages didn't work with it because everyone only cared about IE (...and i'd put the RIIR crowd to shame with my "evangelization" :-P). Nowadays i use Firefox mainly out of inertia.
Perhaps if Google disables most of the stuff adblockers rely on, there will be again a reason to use Firefox.
A year or so after the fact, I can't even remember what I've lost. Couldn't have been that important.
Performance is ok but it isn't my main concern, i'm also concerned about features (after all lynx is faster than both of those browsers, yet it lacks a bit on the features side).
In any case, this isn't a hill i care to die on. I just do not see much of a difference between the two browsers anymore in terms of what they can do. I used to like Mozilla for its features (in fact i was really annoyed that they switched their focus to Firefox back in the day and left what they renamed to "Mozilla Suite" to die) and Firefox later for its (remaining) features. Funny enough now that i think about it, the reason was also "performance" back then and again i didn't care about it but i did care about the features lost.
I guess they kept on the same path and eventually Firefox will be a chromeless, featureless HTML terminal - not very useful as a browser, but it'll be the fastest HTML terminal :-P.
In any case, it still has a number of major differentiation points - more capability for extensions is even one of them, see their Facebook Container extension. But obviously the one Google will never be able to copy is a focus on privacy.