After upgrading to Firefox 15 theme Oxygen KDE -one of the best Topics currently available for integrate the look and feel of Firefox in KDE- It stopped working.
The quickest solution to reuse the theme in the latest version of Firefox was disable plugin compatibility checking browser. There are extensions for this, although it is something that is achieved very easily through the advanced preferences of Firefox.
To disable the compatibility check of Firefox 15 add-ons, first you have to open a new tab and type about: config. We promise the navigator that we will be careful while we rummage through its guts, ensuring that we are not very big hands.
Once inside we secondary click and in the pop-up menu we select New → Logical.
In the window that opens we enter extensions.checkCompatibility.15.0.
And later we select the value "false".
We accept the changes. Now it is enough to go to the configuration section of the add-ons to make sure that the ones that were disabled / blocked for not being compatible with the new browser version they can be activated again.
It should be noted that this is an inappropriate method to use our plugins and that it would be best to wait for them to be updated by your developers. Although if one cannot wait, or needs a specific complement to work, it is well worth it.
More information - Integrate the look and feel of Firefox into Kubuntu, Firefox 15 now available in Ubuntu 12.04