https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

Thanks for your suggestion. None of the software mentioned was installed on my computer. I suspected my own Firefox settings might be the cause and compared another computer that was saving the pop-up exception list. It turns out under Options/Privacy/"Clear History when Firefox closes" (checked)/Settings box, I unchecked the two "data" boxes: "Offline Web Data" & "Site Preferences". The pop-up exceptions list now appears to be saved when Firefox is closed.

In case you use "Clear history when Firefox closes" or otherwise clear history.

• do not clear the Site Preferences

• Tools > Options > Privacy > Firefox will: "Use custom settings for history": [X] "Clear history when Firefox closes" > Settings
• https://support.mozilla.org/kb/remove-recent-browsing-search-and-download-history

• Clearing "Site Preferences" clears all exceptions for cookies, images, pop-up windows, software installation, passwords, and other website specific data.

Hello,

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