This incompatibility with DownThemAll 3.0 Beta 5 appears first with Adblock Plus 2.6.3 - with older Adblock Plus versions all was ok.
And since in marius7x's addons-listing is no DownThemAll I think there are incompatibilites with some more addons and the cause is due to Adblock Plus.
I tested this issue with a brand new profile only with DownThemAll and then I started with adding Adblock Plus 2.4 and go further with adding newer Adblock Plus versions. When it comes to version 2.6.3, the bug appears and the Adblock Button disappears with each newstart.
Therefore in my daily firefox profile I deinstalled version 2.6.3 and went back to 2.6.1 and stopped updating - no more trouble now!
Please fix this bug, so that I can update Adblock Plus again.