Pixelated letters

I recently updated my GPU to rx 480 and got the AMD Crimson graphic driver (used AMD Catalyst graphic driver before).
Since the update it happens frequently that the letters on the Warmane page are so blurry and pixelated that its hard to read anything.
Im using Firefox and have this pixel problem only on the Warmane page. When reloading the page the letters are sharp for a second and then they get blurry/pixelated again. When switching to the Warmane mobile style the letter look fine.
What I tried so far:
- Zooming in/out
- Turning all settings on/off in Control Panel > Performance Options Visual Effects.
- Turned off hardware acceleration in Firefox settings.
- Changed settings in about:config
Nothing helped so far.

Now I dont know if the problem is related to the Warmane page, firefox, my new graphic driver or my new GPU that might be defect.

Anybody experience a similiar problem?

UPDATE:
Just tested the Warmane page on a different computer. Same problem.
Its Warmanes animated background image (the rainy mountain) that causes the pixelated letters.
When I scroll down so the background image is hidden or when I pause the rainy animation, then the letters get sharp.