We experienced the same problem and resolved it by deleting the 'Mass Actions' block on the offending page.
Hi Guys,
I am having the same thing on moodle 3.5.2 "with all updates applied" and the fix for:
Home / ► Site administration / ► Appearance / ► AJAX and Javascript
works but how do i get it to work the right way and not to have the dropbox for activities as it is only happening on safari, Firefox and Chrome are fine.
Tim
Same problem on my localhost Moodle site: Add an activity or resource not clickable in my Firefox browser, but available in other browsers.
Emptied all Moodle caches to no avail.
Opened (F12 key) the console and found this message: QuotaExceededError!
Went to Browser Privacy/Cookies and Site Data
it read Your stored cookies, site data and cache are currently using (a lot of) MB of disk space.
Went to Clear Data and checked Cached Web Content.
That solved the problem!
Related to this other discussion: https://moodle.org/mod/forum/discuss.php?d=364143