Firefox

I'm not aware of one. But I noticed the other day that mapquest (remember then from back in the day? they're still around) still lists it as "Gulf of Mexico".

Its somewhat trivial nowadays to make a chrome extension compatible with firefox. I bet if you bother the dev of that site, they could get it done fast, especially since it's a relatively simple thing to do via an extension and I highly doubt it's using any WebExtension API's that aren't standardized between chrome and fieefox.

I'm switching to OSM, personally.

For android, OsmAnd is really solid and make editing easy. (Organic Maps is good too, but much less featured, depending on preferences.) I've started updating all the places I frequent and anything near me that I notice. Its actually kind of fun, to be perfectly honest. Its a small, somewhat selfless thing to do that has an impact on others around you.

IMHO, helping improve an open alternative for the community benefit is a far better act of resistance than a chrome extension that could easily be a GreaseMonkey script, aside from providing a bit of messaging.

Plugins will only change it on your end and Google won't care since they still make money farming your data when you use their services.

Best to just try get used to OSM and other Google service alternatives.

I also checked and DuckDuckGo's Map also changed to "Gulf of America" might be time to give SearXNG another try.