Please explain the problem in detail. What happens? What is/are the exact error message(s) ?

You can rename/remove search.json.mozlz4 (and possible search.json, search-metadata.json, search.sqlite) in the Firefox profile folder with Firefox closed to rebuild the search engines registry.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Firefox will rebuild search.json.mozlz4 from the builtin search engines.

I installed the version 81.0 yesterday and now i can't do any search because i can't choose a search engine.

there is no error message only what's on the picture i added on my first message

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Go to that message, and use the Undo link to its right.

thanks Cor-el, i did what you said and it works now :)