This is currently not possible (you can only remove all logins). There is a bug report to add this feature, but isn't actively worked.

• Bug 1208194 - Implement multi-select deletion of logins

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If you are familiar with editing a JSON file then you can remove them from logins.json

You can use this code in the Browser Console to get all logins. Remove the logins and then import the modified logins using a fresh logins.json. Make sure to keep a backup. You can possibly open the modified logins file in a Firefox tab to verify that the file is valid JSON.

• "3-bar" menu button or Tools -> Web Developer
• https://developer.mozilla.org/en-US/docs/Tools/Browser_Console

/* export logins in JSON format */
var lg = prompt("Logins",JSON.stringify(Services.logins.getAllLogins()));
/* import logins in JSON format */
if (lg = prompt("Logins JSON: [{},{}]","?")){
lg = JSON.parse(lg);
try {/*[60+]*/
 for(i=0;LG=lg[i];i++){if(!LG.origin){LG.origin = LG.hostname}}
 ChromeUtils.import("resource://gre/modules/LoginHelper.jsm");
 LoginHelper.maybeImportLogins(lg);
} catch(e){console.log(e);}
}

Another possibility is to export the logins to a CSV file and edit this file. You can rename the current logins.json file in the profile folder to loginsOLD.old and import the edited CSV file.

• https://support.mozilla.org/en-US/kb/export-login-data-firefox-lockwise

See the note at the bottom of this article about how to enable CSV import via the about:config page.

• https://support.mozilla.org/en-US//kb/import-login-data-file

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