Thanks for getting back to me and trying to help,...just opened site in private window ,..same thing,...navigating through html pages of the site I still see very short flashes of white,...like pages are "jumping" ,....!
There is solution o local level ( on my Firefox browes to change background in black,....but this is not solution!
Anyway,...thanks a lot once again, for your time,....!
I know for sure that it's not weebly issue but Firefox,..and I'm not the only one who is complaining ! You can find tonne's of discusions and possible solutions but ,....does not work !
All the best, Tim
Maybe some more people will test it out and we can try to narrow down the cause. 
