1. Check Firefox Settings Make sure that your Firefox settings are not affecting the display of open windows:

   • Window Management: Sometimes, the way windows are managed can affect how they are displayed. Ensure that you are not using any settings or extensions that modify window behavior. 

2. Update Firefox Ensure that you are using the latest version of Firefox, as updates can fix bugs and improve functionality:

   1. Click on the menu button (three horizontal lines) in the upper right corner. 
   2. Select Help and then About Firefox. Firefox will check for updates and install them if available. 

3. Restart Firefox Sometimes, simply restarting Firefox can resolve display issues:

   1. Close all instances of Firefox. 
   2. Reopen Firefox and check if the scrollbar for open instances appears. 

4. Check Display Settings Since you mentioned using multiple monitors, check your display settings:

   • Windows:
       1. Right-click on the desktop and select Display settings. 
       2. Ensure that the scaling and resolution settings are appropriate for both monitors. Sometimes, mismatched settings can cause display issues. 
   • macOS:
       1. Go to System Preferences > Displays. 
       2. Check the arrangement and resolution settings for both monitors. 

5. Disable Hardware Acceleration Disabling hardware acceleration can sometimes resolve display issues:

   1. Open Firefox and click on the menu button. 
   2. Select Options (or Preferences on macOS). 
   3. Scroll down to the Performance section. 
   4. Uncheck the box for Use recommended performance settings. 
   5. Uncheck Use hardware acceleration when available. 
   6. Restart Firefox. 

6. Reset Firefox If the issue persists, you can reset Firefox to its default settings:

   1. Click on the menu button and select Help. 
   2. Click on More Troubleshooting Information. 
   3. Click on the Refresh Firefox button. This will reset Firefox to its default state while keeping your essential information like bookmarks and passwords. 

7. Check for Conflicting Software Sometimes, other software (like screen recording or remote desktop applications) can interfere with how applications display on multiple monitors. Try closing any unnecessary applications to see if that resolves the issue.

No, none of this works.

There's a bug in there somewhere.

I wonder if I need to reinstall Firefox?

I wonder if, when a certain number of open instances are reached, this problem occurs?