What do you want them to do? Firefox categorically refuses to support things like PWAs.

They had to make an extension for this to work in Chrome, so the browser's base capacities shouldn't be the limiting factor.

Building a Firefox extension would be the right move.

I wish Google wouldn't force us to use chrome. Hell they have Google Play for games on Windows, they could just offer the complete app store this way and get a close to native app.

How much effort should Google put into supporting browsers that don't have the feature sets to support the use case?

Depends, are the serious about avoiding antitrust cases?

I think Microsoft Office's mere existence precludes Google ever being considered a monopoly in the office apps space.

Mere existence of a competitor doesn't preclude an anti-trust case for monopolization or abuse of a dominant market position.

Lots of office suites exist, but Microsoft has to tread lightly. In Recent News, the EU is dinging them for bundling/tying Teams with Office. Because it's a dominant market position, regardless of competitors.

If at some future time, Google Sheets has a significant market share, they might get dinged, too.

How many browsers are there with more than 2% global user share ?

We're talking about 4 or 5 browsers, putting serious effort into supporting them all would be reasonable.