So, it was pointed out to me that Firefox 25+ has a new, re-designed findbar that breaks my Clear Fields extension – naturally (thanks Luís). I guess I should be happy though, since this is the first incompatibility since Firefox 4 
Support for this new findbar was a bit tricky, but in the end it works again. Version 4.1.0 contains the fix and should be available through Mozilla as soon as it clears their review process.
You can also download the new version by clicking the link below.
Cheers