I finally managed to do this with all my Browsers, Firefox, Chrome, Edge. THESE ARE THE STEPS I DID:
STEPS:
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1) I installed the "Authenticator by mymindstorm, Sneezry" Extension/Addon on all of my Browsers

2) On my browser I clicked the "authenticator Addon" at the top of the Addon/extensions Toolbar, and click the PENCIL (✏) icon, and then I clicked the (➕) icon.

**3) then I choose the [import QR images].

4) Now I take my android phone and I open the Authenticator app on my Phone, and I clicked on the 3 horizontal lines [≣] on the upper Left of the screen.

5) I clicked "Transfer Codes", then "Export Codes"

6) then in the "Select Codes" chose the codes you want to Export in a (QR code). Then click NEXT.

7) I make a screenshot of the QR code that I see on my phone screen.
This step is to send this screenshot of the QR code to your PC. (You can use any way you prefer to send it)

**8) Now Back to the Steps Number (2-3) In the PC browser, in the authenticator, I click the [import QR Images], and then the [Import QR Image Backup] button.

10) I find the screenshot of the QR code I brought from my Phone, I chose it and I will get a "SUCCESS" message on the screen.

That's it! I hope my steps will help many. ✌️😊

You kind of have to question the security of using an authenticator on the same device... but I needed to do that anyway and this served its purpose very well. I used it with Chromium originally, and it was very easy to transfer over to Firefox using the backup system. The fact that it's open source is amazing!

¡Extraordinario de no creer, para no depender de un maldito teléfono; eso era lo que estaba buscando, pero mejor escanear el código QR con la app de un móvil al mismo tiempo, para quedar con un respaldo escrito en piedra!

Great OPEN SOURCE extension. Does everything I need for creating TOTP. Haven't tried HOTP yet and although it has may extended features , not sure I would trust the cloud with my secure secrets. Comes with sensible warnings for you own security, follow them.

This was a great addon, sadly it seems to be discontinued. I wrote a similar one to realize the most of its features, but simpler and prettier. Check it out: Vault2FA

Backup nags and minor annoyances, otherwise perfect.

I really like this addon. However, I noticed a bug: For some time now, when creating a backup as a TXT file, the file is incomplete and many entries are missing. This didn’t happen before, but I started noticing it at some point.

very high CPU usage for no reason

Love it. Can't imagine Firefox without it.

Authenticator.cc is one of those add‑ons that does exactly what you want without overcomplicating things. The interface is clean and simple, making it easy to scan codes or manage multiple accounts without extra clutter. What really makes it stand out, though, is how effortless the import and export process is. Whether you paste in plain text otpauth links or drop in a JSON backup, it just works — which is something many other authenticators stumble on.

Another strong point is that everything is stored locally in your browser profile. There’s no forced cloud sync or hidden upload, so you stay in control of your secrets. That said, it does mean you need to back up your codes yourself, since Firefox Sync won’t carry them over automatically. For some, that might be a downside, but the add‑on balances it out by offering encrypted backup options you can save to Google Drive or another service.

Overall, Authenticator.cc is lightweight, reliable, and refreshingly straightforward. It pairs well with a dedicated mobile app like Aegis for long‑term storage, while giving you the convenience of browser‑based codes when you need them. It may not have the flashy UI or advanced grouping of bigger apps, but if you want simple, open‑source, and import‑friendly 2FA in Firefox, it’s hard to beat.

With a Mozilla account, you can have your two-factor authentication synced across all your devices using Firefox. It works perfectly. Looking forward to the mobile version!

where is the frickin quthenticator now that i've installed it?? why all the secrecy

Отличный аутентификатор. Полная альтернатива смартфону. Пользуюсь больше года.