I recently found that I could not edit WordPress posts, but managed to do so when I used Internet Explorer rather than Firefox.
I have now tried again with Firefox, with the NoScript add-on disabled, so it seems that the culprit was NoScript. There must have been something wrong with a recent update. It also seemed to have weird effects on some other sites, with menu options taking one to the wrong place etc.
It’s a pity, because I found NoScript useful when visiting news sites and others with bandwidth-hogging video streaming etc.