There's a website I use where I frequently middle click the submit button to submit forms to new tabs, so this add-on is essential for that.

The main issue I encountered is that this was triggering the popup blocker in Firefox 57. I had to add the particular website to my popup whitelist to get middle clicking buttons working as desired.

+ opens forms in foreground tabs
+ e10s compatible

- I can't get it to open forms in background tabs (that's my usual usecase - click multiple formsubmit buttons on the same page, rapidly)