It is possible to use the Browser Console to import logins in JSON format, so if your password manager support this format then it is not that difficult. Otherwise you first need to convert your exported password file to JSON.

logins: array of comma separated logins in JSON format:

• logins = [{"username":"xxx","password":"xxx","hostname":"https://xxx","origin":"https://xxx"},{}];

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

See also:

• [/questions/1258644#answer-1220664] How do I export "saved logons"?

That said, how does Mozilla feel about data portability while disobeying the General Data Protection Regulation?

Does GDPR require that locally installed software be able to export data you've stored in it? I don't remember that being a requirement.