You provided no information about installed extensions.
Do you have any extensions installed?

Does Firefox do that while running in the Firefox SafeMode?
http://support.mozilla.com/en-US/kb/Safe+Mode

there aren't any. I've also tried safe mode - no difference

Sometimes it's helpful to rename the settings file that stores the last window size and some other settings (e.g., toolbar customizations). If you would like to try this:

(1) Open your personal settings (AKA Firefox profile) folder

Help > Troubleshooting Information > "Show Folder" button

(2) Switch back to Firefox and exit the program (e.g., Firefox > Exit or File > Exit)

(3) Wait a few moments for Firefox to finish updating files in your settings folder, then rename localstore.rdf to something else, like localstore-bad.rdf or localstore.old. Alternately, you can delete the file.

(4) You're done with your profile folder and you can restart Firefox now.

Does it make any difference?

I've renamed, deleted and changed this folder at least 3 different times. It makes no difference.Thanks for the thought anyway.

I may have assumed something incorrectly. When you are pressing Full Screen, are you having problems with Firefox's full screen feature for the browser, e.g., the one you switch into and out of using one of these:

F11 key
orange Firefox button > Full Screen
classic View menu > Full Screen

or are you having problems with the full screen feature of a plugin, such as the Flash player?

Whether clicking full screen under "view", F11 or the full screen icon I added to the navigation bar, the firefox screen changes from normal view to full screen, but the full screen is only 90% of the screen and off-centre, too. I'm not referring to any plugins. In fact, when I do the above, the status bar on window's taskbar stops showing the "firefox" icon and nothing shows at all,or just the firefox icon without the word firefox next to it, as if nothing is open. If I click the start menu I can get it to show the open icon and firefox name again. When this happens, it almost looks like there is another screen or something open behind the firefox screen, but I can't get at it. The screen won't move and if I press "maximize" or undo "full screen" it goes away. PS: I appreciate you keeping at this.

It sounds as though Firefox might computing the screen dimensions incorrectly.

Could you open the web console (Ctrl+Shift+k), paste the following next to the caret (>) and press Enter to run it? If you could try this when Firefox seems to be working correctly and when it seems to be working incorrectly, I'm curious whether you will get different results.

screen.width + " x " + screen.height;

If you do get different results, does there seem to be any logical mathematical relationship between the incorrect and correct width and height?

I did what you suggested.When I did maximize I got 800 x600
Below is the answer when I tried going from normal to full screen.

[16:13:32.130] Error in parsing value for 'filter'.  Declaration dropped. @ http://www.google.ca/:9
[16:13:32.130] Unknown property '-moz-box-shadow'.  Declaration dropped. @ http://www.google.ca/:9
[16:13:32.130] Unknown property '-moz-border-radius'.  Declaration dropped. @ http://www.google.ca/:9
[16:13:32.130] Expected declaration but found '*'.  Skipped to next declaration. @ http://www.google.ca/:9
[16:13:32.130] Error in parsing value for 'opacity'.  Declaration dropped. @ http://www.google.ca/:9
[16:13:32.130] Error in parsing value for 'top'.  Declaration dropped. @ http://www.google.ca/:9
[16:13:32.130] Error in parsing value for 'left'.  Declaration dropped. @ http://www.google.ca/:9
[16:13:32.130] Error in parsing value for 'right'.  Declaration dropped. @ http://www.google.ca/:9
[16:13:32.130] Error in parsing value for 'bottom'.  Declaration dropped. @ http://www.google.ca/:9
[16:13:32.130] Unknown property 'zoom'.  Declaration dropped. @ http://www.google.ca/:9
[16:13:32.131] Unknown pseudo-class or pseudo-element '-webkit-scrollbar-track'.  Ruleset ignored due to bad selector. @ http://www.google.ca/:9
[16:13:32.132] Error in parsing value for 'background-image'.  Declaration dropped. @ http://www.google.ca/:9
[16:13:32.133] Unknown property 'user-select'.  Declaration dropped. @ http://www.google.ca/:9
[16:13:32.133] Error in parsing value for 'background'.  Declaration dropped. @ http://www.google.ca/:9
[16:13:32.134] Expected declaration but found '0'.  Skipped to next declaration. @ http://www.google.ca/:9
[16:13:32.135] Unknown property 'transition'.  Declaration dropped. @ http://www.google.ca/:9
[16:13:32.136] Error in parsing value for '-moz-transition-duration'.  Declaration dropped. @ http://www.google.ca/:9
[16:13:32.141] Unknown property 'border-color-bottom'.  Declaration dropped. @ http://www.google.ca/:9
[16:13:32.142] Expected color but found '#dcdc'.  Expected end of value but found '#dcdc'.  Error in parsing value for 'border'.  Declaration dropped. @ http://www.google.ca/:9
[16:13:32.227] Unknown property 'behavior'.  Declaration dropped. @ http://www.google.ca/
--
[16:13:37.692] Unknown property '-moz-border-top-right-radius'.  Declaration dropped. @ http://www.google.ca/:1
[16:13:37.692] Unknown property '-moz-border-bottom-right-radius'.  Declaration dropped. @ http://www.google.ca/:1

I don't know if you know what any of this means, but I thought I'd send it just in case.

i was just curious as to if it is your settings in your phone and if you don't mind me asking what kind of phone of do you have so that i may further help you

no phoning

800 x 600 seems way too low. Most monitors now are in the range of 1440 x 900 or more. Is that your actual screen resolution? You can check your current screen resolution by right-clicking the desktop and choosing Properties, then (I think) the Settings tab.

Also, I don't know why you have style warnings when switching between regular and full screen display. The page should not reload or redraw. There's something unusual going on there.

With Firefox closed, could you right-click your program icon, go to Properties, then the Compatibility tab. The settings on this tab can alter how Firefox runs with respect to screen resolution, and also can mess up your Windows desktop at the same time. Try clearing (unchecking) any custom settings on this tab and see whether it makes any difference.

Hi bear58401, as far as I know, this discussion is about Firefox running on Windows 7, not Android.

Oops, I meant Windows XP, not Android.

I have an old 17" square monitor that is always set at 800x600 and until last Thursday, I have never had any problems with firefox on it. I'm running XP not windows 7.

Okay, if you could check what Firefox reads as the screen resolution when it not maximized, just before and just after you go full screen, does it show 800x600 or something else?

whth normal view it shows 800x600. with full screen it shows all those error messages that I included.

Believe it or not, after spending a week uninstalling ,reinstalling, checking configurations, I simply unclicked menu toolbar and now it works perfectly on regular, maximize and full screen. Go figure. Thanks for the efforts,folks.