What can they do? In first line to put the userinterests in front and not the own of the company, this what Mozilla do in last years, apart with their deal with the Devil (Google). The other reason is, that, because Blink is the predominant engine in the web, most pages are optimized for it. A catastrophe if Firefox disappears? I don't think. Over the years, apart from 20 years ago there have only been 3 engines, WebKit, based on the engine developed by KDE and used by Apple, Blink a development based on it and Gecko, developed by Mozilla. There are also other marginal ones that have disappeared due to lack of compatibility with new web standards (f.Ex.Presto). This is precisely also a problem with Gecko, although it can still render the current contents, it does not do it with the efficiency of Blink, which is expressed in several Benchmarks, with a result significantly inferior to those of Blink, which for this reason will take the lead. as a web standard.
Technological developments always converge to a unified format, which facilitates the development and improvements to be done with different systems, this has nothing to do with dominance or monopolies, they are simply practical aspects.
"It will be very bad, if one day the metric system prevails in world technology." https://www.latimes.com/archives/la-xpm-1999-oct-01-mn-17288-story.html