The only reliable way that I've come across for finding stuff after a long time has passed is saving every sightly interesting webpage to Zotero and using fulltext search afterwards (including webpage body).
I'm curious, do you find the builtin browser history facilities sufficient for your needs, or are you using some third party tool for that?
Mostly though I realize I have focused heavily on not having clutter vs. being able to recall quickly everything I've ever found necessary or useful. It's a trade off I like, but it may not be for everyone.
I mentioned this below, but check to see what your history limits are in Firefox (https://support.mozilla.org/en-US/questions/1039372). It's possible if you do enough browsing that you might have trouble finding older pages because they're not there anymore.
I'm not sure what the best mitigation is for that, I've kind of accepted that history for Firefox is short-term, not long-term. It might be possible to rig up a webextension to save history more permanently, but I suspect it would need to do native messaging I think to do that, and at that point maybe it's better to just do regular copies of the SQLite database.
Relying on Firefox history less also has the kind of minor advantage of allowing you to be more aggressive about cleaning it yourself, which can have a noticeable performance impact in some cases.