How to repair Firefox

With every release of Firefox I find that more and more websites that I use become unusable with Firefox, meaning I have to constantly switch between browsers that are not broken, like Chrome and edge. Before abandoning Firefox altogether I would like to know if it is possible to repair Firefox. The latest site Firefox that causes Firefox to crash is thai-language.com, which returns the completely false and meaningless message "The proxy server is refusing connections Firefox can’t connect to the server at www.thai-language.com". This worked well until the latest 'upgrade' or rather 'downgrade' from Firefox.