The web without Firefox will look practically the same as it does now. Firefox has so little market share, and Mozilla so little influence nowadays that it may as well be a web without Firefox.
I hear people proposing solutions to Firefox's continued downfall all the time, but I simply don't see it scraping back any market share at this point. Chrome won the browser war; Mozilla had every opportunity to see-off Chrome's ascendance back in the late 2000s (Firefox's market share was around 20% back then) but squandered the opportunity. Now the majority of users use Chrome, and beyond Mozilla's ideology or Firefox's underlying technical aspects (which the general population—who Mozilla would need to attract if they want to compete with Chrome—clearly don't care about), what reason is there for users to switch away from Chrome?
With that said, I don't see Firefox going anywhere, nor do I see it switching over to Chromium (which wouldn't do anything beyond fixing Firefox's compatibility issues). Google likely isn't going to stop funding it; not just because they get value from Firefox funnelling its users to their search engine, but because letting Firefox (and Gecko) perish will only focus the eye of regulators on what would be Google's new browser engine monopoly. And Firefox itself has a dedicated userbase, who may complain about Mozilla's seemingly endless stream of questionable decisions, but continues to use Firefox and in turn support Mozilla.