1. Inside Firefox, type or paste "about:profiles" in the address bar and press Enter/Return to load it.
2. Click the "Create a New Profile" button, then click Next. Assign a name like "Test1", ignore the option to relocate the profile folder, and click the Finish button.
3. After creating the profile, scroll down to it and click the "Launch profile in new browser" button.
4. Firefox should open a new window that looks like a brand new, uncustomized installation. Your existing Firefox window(s) should not be affected. Please ignore any tabs enticing you to connect to a Sync account or to activate extensions found on your system so we can conduct a clean test.

Check whether the issue persists in this new profile.
When you have completed the test, you can close the extra window without affecting your regular Firefox profile.

Inside Firefox, type or paste "about:profiles" in the address bar and press Enter/Return to load it.
Click the "Create a New Profile" button, then click Next. Assign a name like "Test1", ignore the option to relocate the profile folder, and click the Finish button.
After creating the profile, scroll down to it and click the "Launch profile in new browser" button.
Firefox should open a new window that looks like a brand new, uncustomized installation. Your existing Firefox window(s) should not be affected. Please ignore any tabs enticing you to connect to a Sync account or to activate extensions found on your system so we can conduct a clean test.
Check whether the issue persists in this new profile.
When you have completed the test, you can close the extra window without affecting your regular Firefox profile.

Considering this, it is likely that the issue lies with Firefox browser.
We recommend performing a "New Profile Test" using the following steps:

1. Inside Firefox, type or paste "about:profiles" in the address bar and press Enter/Return to load it.
2. Click the "Create a New Profile" button, then click Next. Assign a name like "Test1", ignore the option to relocate the profile folder, and click the Finish button.
3. After creating the profile, scroll down to it and click the "Launch profile in new browser" button.
4. Firefox should open a new window that looks like a brand new, uncustomized installation. Your existing Firefox window(s) should not be affected. Please ignore any tabs enticing you to connect to a Sync account or to activate extensions found on your system so we can conduct a clean test.
5. Check whether the issue persists in this new profile.
6. When you have completed the test, you can close the extra window without affecting your regular Firefox profile.