Horizontal or vertical?

Opening in the side bar is the default for bookmarks that are created via a link or button on a website. You can check the Properties of a bookmark via the right-click context menu in the side bar (Ctrl+B). In the Bookmarks Manager (Bookmarks > Organize Bookmarks) you can click the More button in the Details pane at the bottom right. Make sure that "Load this bookmark in the side bar" is not selected.

No - my Organize Bookmarks window has no Details pane (Windows Vista Home Premium). My feeling is that this wouldn't be the problem, anyway. Everything was normal up until a couple of days ago, when after a reboot FF started misbehaving. I hadn't done anything out of the ordinary beforehand, and none of my other application windows are affected.

The screen appears horizontally truncated (like an emacs split screen), but any attempt to resize fails. I see from Googling that I'm not the only one experiencing this problem!

Please provide a screenshot of that. https://support.mozilla.com/en-US/kb/Adding+screenshots

http://www.wikihow.com/Take-a-Screenshot-with-the-Snipping-Tool-in-Vista

I tried to reinstall firefox but with no success in solving the problem. Also in safe mode the behavior is the same. I've posted a screenshot of a page. You can see in the top side of the page the actual internet page and in the bottom side a large blank.

You can try "Reset all user preferences to Firefox defaults" on the Safe mode start window.

Create a new profile as a test to check if your current profile is causing the problems
See Basic Troubleshooting: Make a new profile

There may be extensions and plugins installed by default in a new profile, so check that in "Tools > Add-ons > Extensions & Plugins"

If that new profile works then you can transfer some files from the old profile to that new profile (be careful not to copy corrupted files)

See http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

I have found out the problem. There is an extension called FBfun (or something like that) that must be disabled/uninstalled.

Click on Tools -> Add-ons -> Extensions , find the FBfun extension and disable or uninstall it. That's all.

I have this split screen problem and have never had "FBfun" installed...next.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the add-ons is causing the problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).

• Don't make any changes on the Safe mode start window.

See:

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems
• Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems