OK, I could get rid of the SD writing thing that slows everything down to unusability.
As I suspected it was some cashing problem.
I opened up /media/[sD-Name]/pandora/appdata/hdonk_firefox_001/mozilla/firefox/[uSER]/
Here I deleted everything inside ./cash ./offlineCash ./startupCash ./thumbnails
Then I opened up Firefox and
Edit > Preferences > Advanced > Network
1: Delete Cash
2: Override automatic cache management: 0 MB
For me it became a lot faster now.
Hopefully disabling the cache will not slow it down after a few weeks.
As I didn't test much please try and report if it speeds things up for you as well.
Maybe there are some more tweaks, so please post whatever speeds up Firefox.
I'll have to compare the speed with a complete new installation of the appdata folder now.
EDIT:
No need for more testing.
Webpages load within a second.
Even HumbleBundle and GOG are as fast as on my big Laptop.
At least for me all Problems are solved.