My memory may be failing me but I think Firefox 3 was also where it started to feel less like the snappy minimal utilitarian “just a browser” it was in versions prior and started to feel more clunky, branded, and product-y, which also didn’t help its adoption in my opinion.
If nothing else it primed users for adopting Chrome, which originally had a less branded, more utilitarian feel not unlike early Firefox.
This cycle is somewhat repeating itself though as Chrome has become very branded and product-y. Would love to see something similarly functionally-minded as early FF/Chrome come out and take a big bite out of Chrome’s marketshare.
I think the update experience was a small part of it, but for me the process model of Chrome was the main thing that made me eventually switch. Crappy javascript on one tab would lock up the entire browser -- a thing that just simply doesn't happen in Chrome.
This was particularly problematic because MS struggled MIGHTILY to get Vista out the door. It was a decade of really terrible IE which was pushed out longer due to Vista's poor performance and stability.
Meanwhile, you could install firefox and later chrome and get a relatively modern internet browsing experience.
Ironically, I think the thing that hurt firefox adoption was the switch to the evergreen model. The era of firefox 3->~33 was quiet painful, updating was simply far too intrusive. "You are out of date, click here to download the latest version!". Chrome from the get go was pretty good at making that seamless.