It left a sour taste in my mouth that Microsoft joined Chrome instead of investing in FireFox and helping them advance their multi-process architecture quicker.
Historically, I've heard from various sources that Mozilla may not work at the same pace as some of the large corporations do in upstreaming patches and releases to the code base. This has (according to what I have heard, which may simply rest on rumor/speculation/heresey, mind you) kept a lot of large entities from getting on board.
I've also heard that in building Quantum this is less of an issue, but their code base is a mess, and chromium's is incredibly well documented and clean by comparison.
Anecdotally, I would be inclined to agree with my second statement up front, after just browsing the two, but please compare for yourself