You'd need the PC equivalent of a grandma with dementia for it to struggle running Firefox. Anecdotally, I game with my tab collection regularly with no issues, but here's a more scientific test: https://www.howtogeek.com/how-many-tabs-does-it-take-to-slow-down-your-browser/

But even in that case, just bookmark, save, and/or archive the pages in question? It doesn't make sense for them to maintain servers and code on a service so easily replicated by the browser itself.