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Hello
Sorry, actually speak German.
For me has helped Exit
1 Firefox closed
2 Open the file in Firefox profile pref.js 
where the line user_pref (" browser.uiCustomization.state "
Search in the line nav- bar
after \ " wrc -toolbar -button \ "
this inserted  , \ " window- controls \ "
doing it then look like this
"nav-bar\":[ ....., \ " wrc -toolbar -button \ " , \ " window- controls \ ",.....
stored and firefox started

[REDACTED]

Hello
Sorry, actually speak German.
For me has helped Exit
1 Firefox closed
2 Open the file in Firefox profile pref.js 
where the line user_pref (" browser.uiCustomization.state "
Search in the line nav- bar
after \ " wrc -toolbar -button \ "
this inserted  , \ " window- controls \ "
doing it then look like this
"nav-bar\":[ ....., \ " wrc -toolbar -button \ " , \ " window- controls \ ",.....
stored and firefox started

Wow, it works!!!  Many thanks!! 

[REDACTED]

I also had this problem if installing FF 29 and 30 fresh ( clean ) Avast! Online Security does not work. To solve this problem I installed FF 28 then AOS and proceeded to upgrade FF from within ( FF updater ).

Yes!   

Uninstalling FF 30 and installing 28, then updating does work.  The AOS extension is working again. 

For anyone who is having this problem you can download an older version of FF from the link on this page.

http://kb.mozillazine.org/Go_back_to_an_old_version_of_Firefox

PS :  Many thanks to iroc9555  -  You certainly do rock!