You can do that with the BlockSite add-on. The downside of using an add-on is it is easy to bypass the blocks.

If you want stronger blocks you may need to look at third-party web filtering software. For more details see Content-control software - Wikipedia

Also, and although possibly not related to your problem, I have to remind you that the version of Firefox you are using at the moment has been discontinued and is no longer supported. On top of this, it has known unpatched bugs and security problems. I urge you to update to the latest version of Firefox, for maximum security, stability, performance and usability. You can get it for free, as always, at getfirefox.com.

You may have an extension that has corrupted the user agent:
Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.10) Gecko/2009042316 Firefox/3.0.10 FBSMTWB

You can see the Firefox version at the top and the user agent at bottom of the "Help > About" window (Mac: Firefox > About Mozilla Firefox).
You check (compare) what websites see: http://whatsmyuseragent.com/ - Whats My User Agent?

You can check the general.useragent prefs on the about:config page.
You can open the about:config page via the location bar, just like you open a website.
Filter: general.useragent
If general.useragent prefs are bold (user set) then you can right-click that pref and choose Reset.

See Web sites or add-ons incorrectly report incompatible browser and Find what version of Firefox you are using