Initial impressions were good; it seemed snappy, I liked the UI, and it was using less memory than Chrome with the same number of tabs open. A couple of days in things went downhill.
On Mac OS I was getting short freezes every ~10 seconds. I found similar complaints ranging over a period of nearly five years, but no clear resolution. This morning I had FF lock up multiple times while editing Jira tickets. Since FF is still a single process, the entire browser was locked up and required a hard kill, which cost me work.
So, no thanks mozilla, back to Chrome. I know this is one person with one experience, but man, this one experience was pretty off-putting. Maybe I'll try again in another decade.