Great addon, hope it is not malware... Firefox dev team is so stupid, it is 2020 not 1999... Why it didn't occurred to them, that browsers need some basic UI to be usable in information age I can only guess... This helps greatly!

EDIT: sometimes it doesn't highlight text, only after x trial :((( Also seems textmarker is causing 100% cpu usage at mozilla start - i5 9600 KF...