Not all Firefox themes and extensions available on the web are properly served by the server so they are easy to add. Sometimes they are offered as regular .XPI (extensions) or .JAR (themes) files.
To install these, you need to either drag the file to the Add-ons Manager (Tools menu, Add-ons) or select File/Open File… and browse for the file.
There’s yet another easier and more logical way: unhide a secret button in the Add-ons Manager that will let you browser for extension or theme files stored in your computer:
- Enter about:config in the location bar to access the advanced preferences
- Look for preference extensions.hideInstallButton
- Double click it to change it to false

Now you have a Install… button in the Add-ons Manager you can use to install local extensions or themes.
Note that the majority of themes and extensions are available from Mozilla Add-ons where add-ons undergo a review process before made available and updates are served through a secure connection. Also, since it is already whitelisted you don’t need to grant further access to other web sites. Remember to always check if the add-on you are trying to install is not already available there.
Thanks to Frank Lion @ mozillaZine Forums.
Wow! Another one of those that makes me scratch my head and ask ‘why isn’t this default?’ I suppose though this is because they rather have people use AMO to install the add-ons.
This also works (and is more practical) with Thunderbird
[...] Source: Mozilla Links [...]
[...] theme file (.jar). To install it, drag the file to the Add-ons Manager (Tools/Add-ons) or use this hidden install button. Related Categories: Theme, Firefox, Articles Related [...]
This is such a big FU***NG waist
Why this “secret button” is exist?? Drag-n-Drop is not such a big deal!!
Get an add-on if you need this!
It’s just more bits which this app takes!
I’m getting this feeling lately that Mozilla dudes are becoming more like M$ (not the m-o-n-e-y… the stupidity), though the upcoming FF3 looks pretty much great (but please keep the old FF theme, FF is like my whole OS!)
Regards.
Simon
why we have this on about:config and we don’t have this:
http://kb.mozillazine.org/Config.trim_on_minimize
that’s a very good Q..
[...] Uncover Firefox secret add-ons install button : Mozilla Links (tags: firefox extension install howto extensions) [...]
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