from mercurial log I see that there were various changes regarding the toolbar icon in Adblock 2.4.1. I'm afraid however that one of these changes introduced a regression, possibly related to how restartless extensions add themselves to the toolbars. If you have installed certain restartless extensions, the toolbar icon of Adblock Plus just vanishes from the toolbar (and can be rediscovered in the toolbox of the customized dialog).
Here are steps to reproduce the bug with DownThemAll! 3.0 Beta 5 which I tested on a clean profile:
- Install Adblock Plus 2.4.1
- ABP positions it's icon automatically as the rightmost item of the default toolbar (besides the home button)
- Install DownThemAll! 3.0 Beta 5 from the link above
- Position the DownThemAll! "Manager" icon to the left of the ABP icon
- Open a new window (CTRL+N) ---> the DownThemAll! icon is still there, but the ABP icon is gone
- Close both windows and restart Firefox ---> the ABP is gone for good and also doesn't reappear
