I've updated to the latest version of Firefox (75.0 64 bit, according to the help menu), and cleared my cache, but keep encountering the same error in Twitter. When entering a post, if I add an emoji, the text entry interface stops acting in a predictable way. I can't consistently select my place within the post, sometimes I can't add content, I usually can't remove content (although sometimes I can, and sometimes removing the emoji fixes the issue, sometimes not), sometimes if I try to type or click on the text entry area, it pastes random sections of the post back in random places, even though I have not copied those sections to my clipboard. I cannot intentionally copy or paste. I can still interact with things outside the text entry box (to close said box, for example). Other browsers have no difficulty with Twitter's web interface, but I can consistently reproduce this error every time I try in Firefox. My only add-ons are 1Password, uBlock Origins, and Facebook Container.