In case you are using Clear Recent History or clear history otherwise: *do not clear the Site Preferences *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. Make sure that you do not run Firefox in permanent Private Browsing mode (Never remember history). *https://support.mozilla.org/kb/Private+Browsing *Tools > Options > Privacy > Firefox will: "Use custom settings for history" *Deselect: [ ] "Always use Private Browsing mode" ---- Note that your System Details list shows that you have preferences that were used in previous Firefox versions and that are no longer supported in the current Firefox release and preferences that have been changed from the default. You can delete/rename the prefs.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in the current Firefox release. *http://kb.mozillazine.org/Resetting_preferences

Found out that browser.download.lastDir.savePerSite somehow became a string value rather than a Boolean value. Fiddling with it in prefs.js did the trick.