Rated 4 out of 5 stars
Tested on a fresh profile:
About the fact that it brakes the possibility to customize your navbar, it's true and it's not a conflict with another add-on. It happens if after having customized your navbar, you use a restart command from the addon manager (or another add-on or userscript button). Then when firefox reopens, it's forgotten what you've just done.
So, when you have made some customization, just remember to close firefox with the quit button then.
The author should investigate why it happens when his add-on is enabled. I have about 100 other add-ons and none has this issue.