1. Evangelize Firefox to people you know any chance you get and show them all the reasons why its better than Chrome (focus on privacy as an important point). Show them how great browsing can be with a few key extensions (like uBlock Origin, Containers, Privacy Possum, etc.). This does not imply that Mozilla is beyond criticism or that Firefox should not be held to a higher standard.
2. If you can, donate money to Mozilla regularly. [1] As of a couple of years ago (IIRC), more than 90% of Mozilla's revenue was from its partnership with Google for being the default search engine in many geographies. As long as this remains true, Google will only tolerate Firefox being a viable competitor while Firefox still remains as small to moderately sized competition (to avoid antitrust action). If/when Firefox gains a much larger piece of the market, Google will (as it has been doing all these years) even more forcefully use its money and marketing muscle on all its properties as well as add more tiny annoyances on its properties to make Firefox seem buggy citing tangential things like "web standards" or "pushing the web forward" and other euphemisms that companies like Google use to crush competition.
IMO, there needs to be a lot more individual funding of Mozilla while keeping it accountable to users and as per its own manifesto. The level of competition among browsers right now leaves us all vulnerable.
Edit: After seeing a reply to this comment, I did a quick search and found that donating money to Mozilla means donating to the Mozilla Foundation, and that the money may not go to funding Firefox development, which is part of Mozilla Corporation. See this discussion from December 2018 on reddit. [2] It may be desirable for at least part of the donation (as decided by Mozilla Corporation) to go to Firefox development. Does it already happen that way or is there a way to get this result, like in the case of Mozilla Thunderbird where donations go to that project (alone)?
[1]: https://donate.mozilla.org
[2]: https://www.reddit.com/r/firefox/comments/a98gmi/donations_t...