No, extension aren't disabled if you run Firefox in Private Browsing mode. Some extensions and plugins detect Private Browsing mode (Firefox notifies them) and may act differently, but that is up to the developer of the extension. The only thing that Firefox (add-ons not included!) does in PB mode is not saving session data to disk.

I believe you have asked this some days earlier already :-)

Can you attach a screenshot of what you see in Private Browsing mode?

Did you check for other extensions interfering?

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe Mode start window.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes