TBH I always found FF to be fast and zippy, but it seems to me that in recent-ish years the devs have been prioritising on "exciting stuff" like re-arranging the interface (making it more difficult for me to use it the way I liked it; I ended up having 3 or 4 add-ons to bring it back to the way I liked it), and adding crap to it (Pocket, Hello, etc), rather than improving on what were its good points (extensions for example, making sure that they don't get broken with every incremental release) and generally helping distinguish it as a unique product.
The final straw for me was when it was dropped from Pwn2Own for being "too easy", and even though I don't like Chrome much, at least Google doesn't seem to be in the habit of senselessly re-arranging stuff and adding crap, and has a good rep in security terms.