I have an alternative set of explanations regarding why Firefox is seeing a continuous decline for the last 12 years.
1. Mozilla is competing against Google. Numbers are not public but I would be surprised if Google didn't have 10x more people working on Chrome vs. the number of people working on Firefox.
2. Mozilla is competing against Google. Numbers are again not public but I remember reading estimates that the equivalent ad budget for promoting Chrome during year 1 was about 6x the entire budget of Mozilla for that period (writing "equivalent" because webside, Google is its own ad agency).
3. Mozilla is competing against Google. Google owns countless properties besides Chrome, from Google Docs to Google Translate to Android, and leverages all of these (great products) to lead users towards Chrome. Case in point: many properties that don't/didn't work or work correctly with Firefox could be made to magically work if you changed your user agent to Chrome.
4. Mozilla is competing against Google. While Mozilla was front and center on many things open-source, relying on volunteers, Google employs countless (talented) Tech Evangelists and managed to attract considerable goodwill, much of it at the expense of the army of volunteers who used to help Mozilla.
5. Replace "Google" with "Apple" in the above points, adapt product names and repeat.
6. In 2011, predicting that the only way out of this was to outmaneuver Google and Apple on mobile devices/silos, Mozilla bet the farm on Firefox OS and lost. Mozilla never recovered.
7. During the Brendan Eichgate, Mozilla became a hapless victim of the US culture wars, mostly acccidentally. Mozilla never recovered.
Now, I'm not claiming that Mozilla never made any other mistake wrt technology or UX or PR. We've all seen a number of them. What I'm claiming is that these mistakes have next to no influence in comparison to the points above.