Some accessibility tools on Windows might turn off Firefox's multi-process support. Follow these steps to re-enable it:
- Type about:config in the address bar and press EnterReturn.
A warning page may appear. Click to go to the about:config page.
- Toggle the accessibility.loadedInLastSession preference to change its value to false.
- Toggle the accessibility.lastLoadDate preference to reset its value to the default setting.
- Restart Firefox.