WHen that happens, try resetting the page zoom and the text zoom. See: Page zoom Text zoom

The zoom feature can make the fonts bigger but can't stop the web page from moving down to upward. I need to upload a screenshot

Here I am posting the capture of BBC Urdu webpage Which display correct on Internet Explorer. I haven't change the zoom of firefox. Whenever you open this link for the first time firefox will show it correct but later if you brows more page the fonts gets smaller and the web page starts shaking

See: http://kb.mozillazine.org/Window_shakes Firefox keeps jumping up and down

No Luck ! Maybe there is a problem in BBC URDU webpage because I can read another Urdu News website properly.

Update. After I selected clear recent history and selected every thing the website started working normally. Cheers