There is no mass-switching campaign in favor of Firefox. The only foreseeable hope of one happening in the future, is if ManifestV3 kills adblockers, and people decide to switch to Firefox; but now there are so many competitors that oppose ManifestV3 (Brave, Opera, Vivaldi) that Firefox isn't ideally positioned to benefit.
Otherwise, there is no reason to expect the factors behind Firefox's decline to disappear.
Apple bundles Safari with their platforms. Google advertises Chrome on their web properties. Microsoft heavily discourages Windows users from switching away from Edge, and occasional "bugs" reset Edge as the default browser. Most corporations promote Chrome to their employees.
There is no major reason to expect any of this to change. The likeliest change is antitrust action, with "browser choice" screens[0], but I don't see why that would help Firefox more than other browsers.
There's no reason to think that continued incremental improvements in Firefox (the current path) can prevent its decline.
The ballsiest thing Mozilla could do is switch to a forked Blink engine (Mozillium?); they'd save tons of engineering resources which they could refocus on user-facing features & UX, they'd have better webcompat with cutting-edge things (VR, MIDI, etc), they'd still be a part of web standards decisions (since they could still choose how their Blink fork deviates from Google's), and could encourage other Chromium forks to rebase on Mozillium instead of Chromium. But Firefox's most diehard fans would never forgive Mozilla, and they might lose as many users as they gain.
It's hard to think of anything Mozilla can do to double Firefox's market share. Continued decline is the most likely path.