Recently you can observe problems with Joomla in Opera browser and as we checked this comes from compressed mootools library in Joomla 2.5
So if your mootools tabs, accordions, animated menus, image sliders or other extensions don't work correctly, don't worry,
there's a fix for that, just follow the steps:
1) Download two files from
2) Change the names of downloaded files by removing '-uncompressed', like this:
3) Upload changed files into
[joomla_root]/media/system/js/ and confirm replacing existing files.
Mootools should work fine now.