What GPU or Video driver are you using and what O/S are you using?

I'm using a Windows 10 Pc with a MSI GTX 970, driver version 385.69.

Turn Off Hardware Acceleration https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration#w_turning-off-hardware-acceleration

Please let us know if this solved your issue or if need further assistance.

I've already turned it off, but i can not see any difference regarding that problem by turning it off or on. I updated yesterday my driver to the newest version but the problem stayed. Today I tried to change the resolution to FullHD (with borders, no scaling) and the problem didn't show up there. After changing the resolution back to native, the problem was gone. I have no idea why it dissapeared or if the problem was Firefox only (I tried several programs, none of them showed the same issue, but none of them had a such smooth scrolling like Firefox). Before I bought this screen I had a FullHD monitor and i performed a 'hot swap'. If i had to guess, I would say Firefox didn't recognized the new resolution properly and simply assumed i was still using FullHD, and changing the resolution probably changed some settings.

So my problem is back again, Firefox doesnt keep the setting. After I restarted the PC, the issue was back. Changing the solution to FullHD and back to native solved it, but at restart it gets lost. Is there any settings I can change manually to change Firefox's settings permanently?

So the FullHD and Native is the from the Nvidia setting or where are you getting that? I have W10x64 FCU 1709 GTX1060 6G and don't have that issue your describing. I think you might have a driver issue did you uninstall the driver and reinstall it and do "custom" install and uncheck "Nvidia" experience and restarted and see what happens. Oh I have FF57.0.1x64 version installed.

I'm switching the resolution from FullHD to 1920x1200 (native resolution) in the windows system control with the FCU 1709 x64. As I said in a previous post, I updated the gpu driver yesterday, and i selected clean install at the installation, so i dont think that a clean install would change anything. Firefox is the newest version installed, so i have FF57.0.1x64 too. Somehow i cant imagine that it is a driver problem, as far i can see the only issue i have are that small vertical freezes in Firefox with smooth scrolling, everything else is working properly.

I have a LG 32" 1080p Not had any problems with any thing, swapped out for 32" 720p and had to play in Windows. Then capacitors fixed on the LG and had to scale in AMD Radeon software though can scale in Windows also.

There are trouble shooters in Win10 have you tried any or looked for the settings in Win10, am sure seen scrolling in there.

If decide to fail this... take it back and have them set it up on a Win10 computer and see if duplicates issue.

Try giving Win10 a tuneup : Repair System Registry and Files

(FOR DOING Above & Below - WINDOWS All OTHER VERSIONS - Use Search To Find Command Prompt ** Enter CMD.exe to Find COMMAND PROMPT Then Right Click & RUN AS ADMIN)

To run the SFC command use the Windows key + X keyboard shortcut to open the Power User menu and select Command Prompt (Admin). Type "SFC /SCANNOW" (without quotes or copy/paste) and press Enter. For more information on using SFC, please refer to the Microsoft KB: Use the System File Checker tool to repair missing or corrupted system files

Run the Windows Deployment Image Servicing and Management (DISM) and System File Checker (SFC) tools as Administrator. The DISM and SFC tools scan the integrity of the Windows image and all protected system files replaces corrupted, damaged and incorrect versions. (FOR DOING Above & Below - WINDOWS All OTHER VERSIONS - Use Search To Find Command Prompt ** Enter CMD.exe to Find COMMAND PROMPT Then Right Click & RUN AS ADMIN) Type "DISM /Online /Cleanup-Image /RestoreHealth" (without quotes or copy/paste) and press Enter.

Windows 10 To run the DISM command use the Windows key + X keyboard shortcut to open the Power User menu, then select Command Prompt (Admin). Type "DISM /Online /Cleanup-Image /RestoreHealth" (without quotes or copy/paste) and press Enter. Reboot After.

Note : SFC fixes files locally using files found on your system. Note : DISM goes up to Microsoft for new ones. Reboot.

Thank you for the long decription i really appreciate your help. I ran both tools, but it couldnt find any errors, and the issue remained. Just to mention it again, its not a complete freeze or something like that, it's more of some distracting horizontal lines which are not as smooth as the rest and just in the lower part (i guess in the 1200-1080=120 pixels). For running the tools i went into the win10 'splitscreen' and the problem disapeared there. As far i can see it's just firefox fullscreen only.