Search engine data is stored in search.json.mozlz4 in the profile folder, so if you do not keep this data then there might be a problem with this file or there are some settings active that protect the search engines.

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 Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Thanks for your quick reply !

I've tried your recommendations and unfortunetly, it didn't work (disable hardware acceleration or in safe-mode).

But I've done more tests: - with a new profile (mv ~/.mozilla/firefox ~/.mozilla/firefox.bak), it doesn't work either - in safe-mode, Firefox starts in english and not in french (weird but why not...). I tried to add my favorite SE (Qwant) which doesn't exists by default in the english version. Then, I added it a shortcut (q) and restart Firefox in normal mode, and surprisingly, the shortcut remains ! But then, if delete for example Amazon (in normal-mode), the shortcut disapear... - I still can't remove some SE (amazon, ...), even in normal or safe mode

In conclusion, I think there are some problems with firefox's behavior regarding search engines :)

P.S: I didn't tried the Windows version...

Thanks !

Some users have mentioned problems deleting Amazon. What if you just hide it by "unchecking" it in the left column of the "One-click Search Engines" box instead of removing it?

Example Ubuntu user post about Amazon search engine plugin: search engines.

Yes, that works, but it was more a bug report that you can't remove or add shortcut with this environment. I've also tried on Fedora and it doesn't work either but on Windows yes.

Maybe an issue relative to Linux ?