Did Firefox ask if you want to save the new data? Are you using any password add-ons or programs?

https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

Firefox does ask if I want to replace the old password with the new one. But even if I say yes, when I disconnect and try to login again, the old password is used instead.

I don't have any programs or add-on tied to passwords, and I did try with all security disabled on my computer. The strange thing is, the login.json file shows a modified date of 09/03/2019, so the file has no problem writing it seems.

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

If the problem remains, open the Password Manager and remove the data for that site.

The problem is still present in Safe mode, and afflict every site, not juste one, so removing data doesn't seem like a very effective solution on the long term.

If necessary, you can edit the passwords by hand on the Options page:

• Windows: "3-bar" menu button (or Tools menu) > Options
• Mac: "3-bar" menu button (or Firefox menu) > Preferences
• Linux: "3-bar" menu button (or Edit menu) > Preferences
• Any system: type or paste about:preferences into the address bar and press Enter/Return to load it

In the left column, click "Privacy & Security".

Scroll down to the Logins & Passwords section and, on the right side, click the "Saved Logins" button. Then click the "Show Passwords" button.

When Firefox shows the password you want to update, double-click it to make it editable.

As a test, Start your Computer in safe mode with network support. Then start Firefox. Try Secure websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting Any Computer In Safe Mode; Free Online Encyclopedia

If you can't edit the password by double-click the password field in the Password Manager then there might be a problem with the logins.json file. You can check the Password Manager for fields with an empty name or password in case the login data is corrupted or otherwise malformed. You can also try to open the logins.json in a Firefox tab.

Well, the good news is that manually changing the password in the password manager does work, so hurray for no corrupted file I guess.

It will do for now, but I was more looking for a way to correct the bug than ignore it at first though.

Thanks anyway for the help