The first step is for Mozilla to finally stop shooting Firefox in the foot by dumbing it down into oblivion, adding unnecessary stuff nobody asked for or removing features people stayed on Firefox for, angering it's loyal userbase in the process and loosing market share. And then we can talk about what WE can do.

What I can do, is criticize them and hope they listen. I'm not gonna use a product I dislike just of charity, to boost their marketshare, while they're swimming in those sweet Google dollarydoos and ignoring their long term users.

Test your code against Firefox. I've encountered many bugs (mostly small and non-breaking issues) in major enterprise SPAs which happened because the developers only did their stuff in Chrome. Little things, like spans acting as check-boxes (why?) only being selectable via keyboard, or custom tool tips not aligning with elements come to mind. But also breaking issues, like using APIs that only just landed in Chrome or assuming the non-standard behavior of the Chrome behavior is the actual standard across all browsers.

every day new people come here with "i made a new browser extension" that invariably only has chrome support. demand they bring firefox up to speed or give preference to firefox addons.

this is my 2 cents

Stop using Chrome yourself, and help your less-tech-savvy friends and family switch to Firefox. During Firefox installation, select the options to migrate everything from Chrome, then set Firefox as the system default browser and uninstall Chrome.