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This is how you reenable Session Manager after it has been disabled by firefox 57:
Optional: Backup user profile, by copying the folder on hamburger menu-> '?'-> 'Troubleshooting Information'-> 'Open Folder'-> go up one level and copy the folder with random letters and numbers.
1. Install a version of firefox that supports "legacy" extensions, like FF ESR 52.
2. Run the browser. If Session Manager is still disabled, go to this page, Click "Add to Firefox" and install Session manager

Léirmheas ar leagan níos sine den bhreiseán seo (0.8.1.13).  Rinne an t-úsáideoir seo léirmheas ar an mbreiseán seo cheana.