Sometimes those websites lied that they don't support Firefox. For example, google meet didn't support background blur on Firefox? Change the user agent to chrome and it suddenly worked!
As for simple stuff such as menu or elements not loading, it's usually the dev copy pasted outdated code/css that uses WebKit/Bink-specific prefix even though Firefox already support them if they removed the prefix. Nothing we can do about that except pestering the dev to fix it or overriding it yourself using some css overrides extension.
Saying that you use Brave seems to result in instant downvotes in lemmy. Kinda makes sense given the fediverse demographic simply don't align with Brave's CEO.
Also, mentioning Kagi often results in replies accusing ads and shilling because Kagi is a paid search engine (with free trial), but it worth checking nonetheless. They reached 20k paying users recently, not bad for a new paid search engine. The fact they're able to convince 20,000 people to pay for search engine means the search result is pretty good.