Install the regular Firefox for Mobile, then install this addon https://addons.mozilla.org/en-US/mobile/addon/phony/ . You can then display desktop versions of websites.

I believe you would have to change the user agent in Firefox, see http://www.makeuseof.com/tag/load-desktop-version-websites-google-android-honeycomb-tablet/

which is not practical as I'm sure you would prefer to toggle it on or off, since generally the mobile pages are better suited to the device and you have no choice but to view only one window at a time anyway.

Don't overlook the suggestion to use the Dolphin browser on your Android. The Dolphin browser has a Dolphin App "Dolphin Desktop Toggle" the links can be found in the above article.

Sidenote: Dolphin has scribbled gestures which helps with the lack of a real keyboard, especially when holding the device in one hand, which is as close as I can come to a simple keyword shortcut for a specific url or a simple javascript invocation without operands. In other words without keyword bookmarks which provide for substitutions and JavaScript in an actual bookmark there is no really decent mobile browser. Having 500 search engines limited to use as if for search usage is not a practical substitute in browsers in place of Firefox's keyword shortcuts available on desktop systems.

correction:

Looks like "Phony" Firefox mobile extension suggestion fits what was asked for if a permanent change is wanted.

To switch usage quickly back and forth for a page would be a bit awkward having to go through menu then preferences, even to see what choice you are using.

This reply provides some pictures on usage of "Phony".

https://support.mozilla.org/en-US/kb/firefox-mobile-doesnt-support-flash#os=android

dmcritchie, the addon I refenced will automatically change the user agent, allowing you to switch between mobile and desktop versions easily.

Hi tylerdowner, Thanks I did see your reply and installed "Phony".

thanks for both of you especially tylerdowner, phony fix my problem. i can manage my facebook now, but still can`t play game on facebook. it says that i need adobe flash player but i do download it before..

Firefox for mobile doesn't support flash just yet. Once Firefox 11 is released, you will be able to use flash in the browser, but that won't be for another week or so.