You can change your password from your Desktop and then try again.May be it's work.

I just tried changing my password: it still doesn't work.

I am using Orbot (tor for android). Could this interfere with the correct functioning of login to firefox sync?

Ok, just tried without tor: still not working, so it can't be tor's fault.... I really don't know what to try next..... Sync is the real strong feature of firefox for android!

Yeah Sync work in Android.I am already using it. You can try by uninstalling your Firefox. Or you can follow this post and this

I checked both posts (before asking myself, actually). On the help page (second post) I didn't find anything valuable to this issue. Now, after you told me to try it, I created a new sync account with a new email, but this also didn't work :( (still the same incorrect-password error!)

Hi Have you disabled your Cookies in your browser? You have to enable it. Just Clear Private Data from your mobile browser.

Sorry for late reply.

Now I have tried deleting all (cookies, active logins, etc.) private data from my mobile browser and when I tried logging in to sync and it still isn't working! I am really used to working with computers, but in this special situation I don't absolutely know how to tackle the problem and where to start debugging....? No matter for the delay, and thanks for your help!

Now I even tried logging out from desktop (for if the logins were colliding), but even alone it my mobile won't sign in to sync :(

Can you give me a screen shot.What it shows when you try to login.

I added an image before I clicked the sign-in-button and one after. (I pixelized my email, just because this is a public forum)

Have you check by changing your password? use a simple password and try in both mobile and desktop. Otherwise uninstall your browser, delete all your backup data of firefox in your phone(powerfull uninstall).

Right now, having reinstalled firefox and having set the password to a stupid string it works!!! I am starting to believe my qr-coe generator didn't work correctly... That's the way I use to transfer huge random passwords from my desktop to my phone!

I will try changing the password to something serious, but anyway thank you very much!!! :)

Ok, now I even managed to set my absurdly long random password :D I believe the issue was because of a different firefox version installed than the latest one (possibly even from f-droid... I don't remember exactly)

Hey, but really thank you very much for your help Fahim: as stupid as this problem has finally showed itself to be, I wouldn't have figured it out on my own 8/

Best and thanks Taranur

Welcome, Finally it works. Great works Taranur. I am very glad to hear that your problem is solved. Cheers :)