Not technically savvy to understand the exact workings of rate limiting, but I often go through the updates list, and open titles in their own tabs as I go, which tend to result in a lot of tabs. It so happens that sometimes I walk away from the pc for some reason, or take a nap or something, and when I come back and click one of the tabs to check it out, suddenly all the tabs on the mangadex domain refresh? I think that triggers it as I vaguely recall getting a few 'too many requests' in the past. Which is strange as the 'browser.tabs.unloadOnLowMemory' is at default value in firefox(true) so the tabs should be asleep if not accessed for a time?
That's why I often just bookmark the titles then close them and, and go through the bookmark instead when checking them...