Firefox is the most popular browser on the Linux platform, but it’s a GTK app and doesn’t integrate as nicely into KDE (which uses Qt). There are a number of ways to get it to integrate better, such as kgtk, but this is an easier and more stable method.
The Oxygen KDE Firefox addon does a great job of theming Firefox to look like a KDE app.
Simple download it and drag to add-ons. I scanned the extracted files with ClamAV and it was clean, but I always recommend that you do the same yourself for 3rd party add-ons.
Unfortunately it doesn’t modify Firefox to use KDE’s file manager or anything like that (unlike openSUSE’s modification), it’s just a theme. But it certainly looks great!
There are some extra things you can do to make it integrate even better.
KDE file picker
You can replace the GTK file picker with a simple KDE-ish file manager (as pictured above) by changing a setting in your config. Browse to:
Search for ui.allow_platform_file_picker and set it to False. Restart and now you’ll get a simple, but KDE themed file picker.
Install the PlasmaNotify add-on to get Firefox to use Plasma for notifications. Turn off the “Downloads window” under Preferences, for a more integrated experience.
Download with KGet
You can get Firefox to download with KDE’s KGet download program by installing the FlashGot add-on.