Following up comment 17, Mark asked yesterday about how fast I was doing it when the problem reproduced and given that this scenario has also been reported during the update, I've managed to narrow a scenario to reproduce the problem consistently: 1. Install Firefox 69 fr. (probably works with any locale) 2. From about:preferences/Language add another language (in my tests I used fr build with language change to en-us). - this option is available only in release channel, I don't know which preference controls this for the other channels. 3. At this point you should have two languages and when selecting the one that you want to switch to the "Apply and Restart" is available as in the screenshot. 4. Hit "Apply and Restart". 5. Check the search engines. If after the restart the search engines are ok, change again the language and hit restart. Do it a few times and 2-3 changes and restarts will likely mess up the search engine list. Given the above scenario using the change language and the originally reported issue during major version update, I would guess that some kind of race condition happens on initialization/re-initialization of the search engines list.
Bug 1578807 Comment 24 Edit History
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Following up comment 17, Mark asked yesterday about how fast I was doing it when the problem reproduced and given that this scenario has also been reported during the update, I've managed to narrow a scenario to reproduce the problem consistently: 1. Install Firefox 69 fr. (probably works with any locale) 2. From about:preferences/Language add another language (in my tests I used fr build with language change to en-us). - this option is available only in release channel, I don't know which preference controls this for the other channels. 3. At this point you should have two languages and when selecting the one that you want to switch to the "Apply and Restart" is available as in the screenshot. 4. Hit "Apply and Restart". 5. Check the search engines. If after the restart the search engines are ok, change again the language and hit restart. Do it a few times and 2-3 changes and restarts will likely mess up the search engine list. Given the above scenario using the change language and the originally reported issue during major version update, I would guess that some kind of race condition happens on initialization/re-initialization of the search engines list. **Edit: Browser console log with browser.search.log = true: https://pastebin.com/KYJ7dbmn