Websites identify your browser by a "user agent" string. If you click the "More System Details" link in the Question Details section to the right of your question, you will see that your Firefox told this site your browser is:

Mozilla/5.0 (Windows; U; Windows NT 5.1; it; rv:1.8.1.14) Gecko/20080404 Firefox/2.0.0.14 YooBarre

To which I say, what??

This article describes how to dig into Firefox's about:config preferences editor and flush that out: How to reset the default user agent on Firefox.

Can you restore normality?

I have a test page that shows your user agent string if you want to check what sites are seeing: https://jeffersonscher.com/res/jstest.php

Nice browser test.

Your System Details List shows that you have three general.useragent.* prefs including a general.useragent.override to modify the user agent (general.useragent.extra prefs are no longer supported).

1. general.useragent.extra.ALX_NS_PH: AlexaToolbar/alxf-2.14
2. general.useragent.extra.microsoftdotnet: ( .NET CLR 3.5.30729)
3. general.useragent.override: Mozilla/5.0 (Windows; U; Windows NT 5.1; it; rv:1.8.1.14) Gecko/20080404 Firefox/2.0.0.14 YooBarre

You can reset all three of them on the about:config page via the right-click context menu.

• http://kb.mozillazine.org/about:config

You can reset these as well:

1. browser.history_expire_days.mirror: 180
2. browser.places.importBookmarksHTML: False
3. browser.places.importDefaults: False
4. browser.places.leftPaneFolderId: -1
5. browser.places.migratePostDataAnnotations: False
6. browser.places.updateRecentTagsUri: False

Help > about Firefox says it is the 26.0 version and that Firefox is up-to-date. I have Windows XP service pack 3 on my pc (no doubt about this). I don't understand anything you say about config etc. etc. Sorry! I'm not a technician. What happened exactly...why should I need to check configuration?

Hi PaolaC., Firefox allows you or add-ons or external software to modify settings that do not appear in any dialog. The about:config preferences editor is an internal page that you can use to view these modified settings. There also are some files on disk, but I think the about:config page is a bit easier to use.

So, here goes:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste usera and pause while the list is filtered

Generally speaking, any line in bold has been modified from the default value and you can reset that by right-clicking it and choosing Reset. Particularly any bolded lines that contain the word override should be reset.

Then try the browser test page again to confirm that Firefox now identifies itself as Firefox/26.0 instead of Firefox/2.0.0.14.

Any luck?

You can also choose to reset all prefs by going to the Firefox profile folder and closing Firefox and delete the prefs.js file in this folder.

You can use this button to go to the Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)

Note that Windows hides some file extensions by default.
Among them are .html and .ini and .js and .txt, so you may only see a name "prefs" without file extension instead of "prefs.js".
You can see the real file type (file extension) in the Properties of the file via the right-click context menu in Windows Explorer.