Make sure you can see the four rounded corners on the Firefox window. If not move and resize the Firefox window back fully into the the screen. "Alt+SpaceBar" allows you to move the window and to resize

If something is moving the window while you are opening tabs

Tools > Options > Content > Advances next to JavaScript

Disallow Scripts from moving and resizing windows

If you are opening browser windows within the browser window, two extensions I know of and don't know of a problem with either,

Fox Splitter (formerly Split Browser), and Split Pannel.

Some similar: Tile Tabs, and Tile View

MultiSideBar looks like it could be confusing like maybe if you are opening bookmarks that are indicated to be opening in a sidebar.

I tried all the advice above except that I did not understand where to access Tile Tabs and Tile view.

When I open Firefox (I have version 5) my Homepage which is Google displays croorectly in the centre then after a couple of seconds it moves to thje right whith nearly half of it off the screen. As A result the House/Home icon is alos out of sight. There is no way that I can centre the page. The same thing happnes when I open up a new tab excxept that it immodiately to the right. This only started happening the last few days. Nothing had been altered on the computer and I had not downloaded anything nor authorised any changes. I went on to Google via Internet Explorere and found that others were having the same problem.I tried a number of their recommendations but none have worked. I have tried uninstalling version 5 and reinstalling. I have even tried going back to version 4 and the same thing is happening. I have Windows 7 64 bit.

go to View Menu -> Zoom -> click "Reset"

go to View Menu -> Page Style -> select "Basic Page Style"

go to Help Menu -> select "Restart with Add-ons Disabled"

Firefox will close then it will open up with just basic Firefox. If above suggestions don't work then follow these steps:

-> go Tools Menu -> Clear Recent History -> Time range to clear: select EVERYTHING -> click Details (small arrow) button -> place Checkmarks on ALL Options -> click "Clear Now"

-> go to Tools Menu -> Options -> General -> When Firefox starts : select "Show My Home Page" -> Type the address of the website which you want to be your HomePage e.g. http:www.google.com -> click OK

-> go to Tools Menu -> Add-ons -> Extensions section -> REMOVE any Unwanted/Suspicious Extension (add-ons) -> Restart Firefox

Check and tell if its working.

Thank you. That worked. Opening up in Basic Firefox is now fine. But why it happened is a mystery and obvioously a big problem from all the items in Google Serach which showed so many having the same problem. I hope they try out your solution. Thank you agin.

Do you have the Babylon Toolbar 1.1.8 installed?

If so, disable it.

I did have Babylon Toolbar earlier but removed/disabled it. I will bear that in mind if I have the problem again. Thank you. Glad to say that Firefox V5 is now working correctly and the pages are always in the centre.