Could you try disabling graphics hardware acceleration? Since this feature was added to Firefox, it has gradually improved, but there still are a few glitches.

You might need to restart Firefox in order for this to take effect, so save all work first (e.g., mail you are composing, online documents you're editing, etc.). Then perform these steps:

• Click the orange Firefox button at the top left, then select the "Options" button, or, if there is no Firefox button at the top, go to Tools > Options.
• In the Firefox options window click the Advanced tab, then select "General".
• In the settings list, you should find the Use hardware acceleration when available checkbox. Uncheck this checkbox.
• Now, restart Firefox and see if the problems persist.

Additionally, please check for updates for your graphics driver by following the steps mentioned in the following Knowledge base articles:

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Upgrade your graphics drivers to use hardware acceleration and WebGL

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The OP is on Mac, so Firefox hardware acceleration is in Firefox > Preferences > Advanced > General

You can also check the Silverlight settings to see if you can disable hardware acceleration in that plugin.

Turning off hardware acceleration did not resolve the issue.

Silverlight, Firefox, the OS (including drivers) are up to date.

I'm not sure where to check Silverlight settings as I see no gui (everything works fine in another browser with same addon)

No extensions nor themes are installed.

Very strange indeed. Any other ideas?