Dropdowns (top of main page Manga/Follows/Community/Info/Settings/Username, reader settings button on legacy reader, All/Follows 6hr/24hr/7d Follows/Rating Forums/Manga toggles on main page, and many other buttons in various places) also stopped working today in old Firefox.
Clicking them does nothing. I assume it's probably the same issue Steelbuck is seeing on MacOS Safari. Not really expecting old Firefox to be supported at this point, but fixing one would probably fix the other.
FWIW, old Firefox was also having the same missing element issue Steelbuck described the other day, which are now visible, but caused this new problem.
Edit: Still broken in Firefox 45.9 ESR as of 2020-04-28 8:26PM PDT
Edit2: All issues appear fixed in Firefox 45.9 ESR as of 2020-04-29 5:57AM PDT
Edit3: All issues broken again in Firefox 45.9 ESR as of 2020-04-29 11:15AM PDT
Edit4: All issues appear fixed in Firefox 45.9 ESR as of 2020-04-29 11:33AM PDT
Edit5: All issues broken again in Firefox 45.9 ESR as of 2020-04-29 11:37AM PDT