In sites such as Instagram and LinkedIn (these were the ones I noticed this happening), after I close an overlay containing a post and its comments by clicking outside of it, it starts selecting everything, as if I'm holding the mouse button. Clicking again stops selecting. And as fas as I know, this is only in Firefox (doesn't happen in Chrome nor Edge). I've tried disabling all extensions, and it still happens. I've also posted this as a reddit thread here, with a video attached showing the problem happening: https://www.reddit.com/r/firefox/comments/19d2fv6/in_firefox_everytime_i_close_an_overlay_it_starts/