Let me pay for Firefox

(Native German speaker here), it's a very rare use of the word "free" and usually only used in fixed terms like "Freibier", it wouldn't even work for other drinks, e.g., you can't say "Freisaft" or "freier Saft" for free juice, it has to be kostenlos there.

The other Wiktionary example of "freie Krankenversorgung" sounds wrong to be, but it seems to be used rarely in some more formal or legal contexts, no one would say it like this in a casual conversation. Google results also show a 4x difference between frei and kostenlos here in favor of kostenlos. But both are low since "Krankenversorgung" is already a very unusual word. I suspect many of those uses might be bad translations from English.

"Freier Eintritt" seems to be used quite often.

“Frei” can have the meaning of “kostenlos” (https://www.dwds.de/wb/frei#d-1-1-7), but these are limited circumstances that are usually perceived as metaphorical idioms. “Freie Software” has no direct connotation of being “free as in beer” (unlike “Freeware”).

I think in some way it has become that, but I assume the roots are different. People might have said "Freier Eintritt" before it became associated with money. One might be able to see this in "Portofrei" which does not mean "free as in beer" but no postage required - for my feeling it doesn't feel like "Freier Eintritt" yet, it does not have as much money connotations (though when it wanders to the front like in "freiporto" it feels more to be about money).

I do think there is a spectrum. Funny things like "Freifahrt" or even "Freifahrtschein", or "Freikarte", or "Freiexemplar", "Freiparken", "Freiminuten" or "Freivolumen" (people might use "Inklusivvolumen") - so I'd argue when used as part to form a new word it is a synonym for "kostenfrei" (not yet in "Freiwild" which changed a lot).

The use as a suffix "-frei" is something different I think. I'm pretty sure it has kept its meaning of "-los" ("-less") since a long time ago.