macOS Big Sur with multiple displays: Firefox tab crashes with Multi-account containers

[waako]

• Multi-Account Containers Version: 7.4.0
• Operating System + Version: macOS Big Sur 11.6
• Firefox Version: 92.0.01
• Other installed Add-ons + Version + Enabled/Disabled-Status:
  • 1Password extension (desktop app required): 4.7.5.90, Enabled: true
  • Always In Container: 1.0.6, Enabled: true
  • Amazon.co.uk: 1.9, Enabled: true
  • Bing: 1.3, Enabled: true
  • Chambers (UK): 1.0, Enabled: true
  • DuckDuckGo: 1.1, Enabled: true
  • eBay: 1.3, Enabled: true
  • Firefox Multi-Account Containers: 7.4.0, Enabled: true
  • Google: 1.1, Enabled: true
  • RescueTime for Firefox: 3.2.10, Enabled: true
  • Wikipedia (en): 1.1, Enabled: true
  • Assistant RGAA: 1.0.4, Enabled: false
  • axe DevTools - Web Accessibility Testing: 4.17.1, Enabled: false
  • Checka11y.css: 1.2.1, Enabled: false
  • Container Hotkeys: 1.3, Enabled: false
  • Facebook Container: 2.3.1, Enabled: false
  • Flagfox: 6.1.43, Enabled: false
  • Google Container: 1.5.4, Enabled: false
  • HeadingsMap: 3.10.1, Enabled: false
  • Ninja Cookie: 0.7.0, Enabled: false
  • OneTab: 1.54, Enabled: false
  • Stylus: 1.5.21, Enabled: false
  • Tampermonkey: 4.13.6136, Enabled: false
  • WAVE Evaluation Tool: 3.1.3, Enabled: false
  • Web Developer: 2.0.5, Enabled: false
  • Xdebug helper: 1.0.6, Enabled: false

Actual behavior

Since macOS Big Sur update and using multiple displays,
Firefox Multi-Account containers causes tabs to repeatedly crash and containers toolbar selector to not open. Still possible to open new tabs in a container by using right click on a tab.

Tested with all add-ons except Firefox Multi-Account containers disabled, same problem.

Expected behavior

Tabs to not repeatedly crash and for containers toolbar menu to open.

Steps to reproduce

1. Install Firefox Multi-Account containers
2. Use macOS device with multiple monitors (laptop + monitor, or dual-monitors)
3. Open new tabs, wait for first tab(s) to crash
   
4. Then click on containers icon in toolbar, nothing happens
   
5. Tabs then crash repeatedly, not all but most.

[dannycolin]

Do you still experience this issue? It seems strange that these crashes are related to Multi-Account Containers.

If you still exprience this problem, go to about:crashes after it happens and search the crash report here if there's one.

You can also try to create a fresh profile and reproduce the issue in it with only Multi-Account Containers installed.