Can you file a bug report please. Also expand on the issue as it has to be duplicated, so size of screen ..... https://bugzilla.mozilla.org/ To submit suggestions for new or changed features, may I suggest: Feedback: https://qsurvey.mozilla.com/s3/FirefoxInput/ Facebook: https://www.facebook.com/Firefox Reddit: https://www.reddit.com/r/firefox/ Twitter: https://twitter.com/firefox Please let us know if this solved your issue or if need further assistance.

I've just found a bug already reported and actively been dealt with: https://bugzilla.mozilla.org/show_bug.cgi?id=1360278

So, I went to my about:config and changed ''ui.context_menus.after_mouseup'' to ''true''