Thanks Alzander!
You are the kindness in person =)
Of course i'm not satisfacted at all because popup is helpful to show users they have to wait few seconds before login (connection sometime is slow and users don't know what's happening).
Anyway i understand this issue was not affecting Jfbconnect.
I think problem is that Joomla 1.5 is not anymore full supported (and i can't do a migration because several components were paied a lot of money and are native for J1.5).
Problem is that not all programmer (like you) can grant a GOOD compatible list for their soft (i'm almost sure that on Joomla 2.5 i could not have these issues).
For now i'm happy that all can still work better as possible in my website
And i hope Jfbconnect will still support J1.5 in future (as long as possible... then i will start from begin with another cms. Too many changes and incompatibilities between joomla versions)