86% of revenue of Mozilla corp are royalties, which encompasses any search engine that shares revenue with Firefox, not just Google. But the remaining 14% is not only revenue generated by Firefox, but also by e.g. Pocket. So from their financial statement, it's pretty hard to say what percentage of Firefox's revenue is derived from Google.
This year there was a post here on Firefox overtaking chrome in speedometer 2, so there certainly is measurable progress. Another option is to look at the interop2023 effort and how many test failures were fixed in Cross-Platform tests.
Just a small correction. Large parts of revenue are obtained though sharing ad revenue between Mozilla and a search engine, not for being the default search engine.
86% of revenue of Mozilla corp are royalties, which encompasses any search engine that shares revenue with Firefox, not just Google. But the remaining 14% is not only revenue generated by Firefox, but also by e.g. Pocket. So from their financial statement, it's pretty hard to say what percentage of Firefox's revenue is derived from Google.