This is likely a graphics driver problem. Disable hardware acceleration and see if the problem persists:
Pale Moon -> Settings -> Advanced -> Use hardware acceleration when available
If that fixes the problem please let me know which model Mac and specific MacOS version you are using so I can add a blacklist for your driver.
Pale Moon -> Settings -> Advanced -> Use hardware acceleration when available
If that fixes the problem please let me know which model Mac and specific MacOS version you are using so I can add a blacklist for your driver.