But one minor annyoing thing Rated 4 out of 5 stars

This is really a great add-on. It unifies the filter and the search so you have no redundancy with the search boxes, and you can either filter your e-mails or, if you hit enter, search everything for a specific word. Just wonderful.
Only annoying thing is, that when you backspace away the last character, the search box immediately loses focus. Thus it happened to me VERY often that I accidentally archived away e-mails, because I though I was still typing in the search box. Unfortunately I couldn't find any option to make this go away. Maybe I've just overseen something, I don't know, but it would be great if that one thing, and I presume it shouldn't be too hard to fix, could be addressed soon.

Thank you for a job well done on this add-on!

This review is for a previous version of the add-on (0.6.4). 

Thanks Qudeid for your feedback!

I tried the problem you report and I couldn't reproduce it. I have another version developed already but I couldn't upload it to AMO (here) still. It's on https://github.com/IagoSRL/TBX-Unified-Search/tags if you want (rename the file from .zip to .xpi and manually install it to Thunderbird selecting the option 'install from file..' on Add-ons tab).

If the problem still continues, please let me know it, here or on my email, twitter or opening an issue on https://github.com/IagoSRL/TBX-Unified-Search/issues