Hi
Just an early "Heads Up" (nothing more) for FDM Developers.
Based on today's Beta Candidate: Firefox 40.0b Candidate Build 2 (my PC uses O/S Win7 (x64)

The status for my large mix of 30 Firefox Add-ons after installation:

25 OK and accepted by Firefox's Extension Signing

4 HAVE CAUTIONARY WARNINGS BUT PERMITTED TO WORK. They are not yet digitally signed. They are displayed with a beige & white diagonal background.
These Add-ons have a Cautionary Brown message (which can be viewed by clicking on the Add-on's "More" link):
" xxxxxxxxx could not be verified for use in Firefox. Proceed with caution
Purely for those interested these are:
SearchHotKeys 1.0, (does not appear on Firefox's Offcial Add-ons site)
HTTPS - Everywhere 5.0.5,
Ublock Origin 0.9.9.2 (forked version by original author (I noted uBlock is signed),
YouTube Center Developer Build 525 (I'm currently using Development pre-release version.
The Official release of YTC 2.1.0.1 is Extension signed, but has not been updated since Jan 2014 and has been reported as having some issues from Firefox 38.0.5 Official Release) I know the author is very active with Dev. Builds but it looks like he needs to get one of them to Release PDQ, if Users are to be able to continue using his very nice add-on much longer)

1 ADD-ON HAS BEEN AUTOMATICALLY DISABLED
Free Download Manager Plugin (1.7.5.2) which is displayed at the bottom of the Add-on List with a pink & white diagonal background, together with a Red Message:
Free Download Manager plugin could not be verified for use in Firefox and has been disabled

I have NOT yet checked to see if Firefox 40.0b Candidate Build 2 will allow me to disable Extension signing as I have only a hour ago installed.

Hope above is of interest
Regards, Dave