You asked about others having problems. Your issue seems similar to another Mac 10.5.8 user:

• https://support.mozilla.org/questions/928804 Just updated to 13.0 on my mac but now I do not have tabs or see my open websites

In any case, this article helped solve the problem the other thread: Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Do you mean that a specific web page opens when you start Firefox?

• https://support.mozilla.org/kb/Firefox+has+just+updated+tab+shows+each+time+you+start+Firefox
• https://support.mozilla.org/kb/How+to+set+the+home+page - Firefox supports multiple home pages separated by '|' symbols
• http://kb.mozillazine.org/Preferences_not_saved

Nothing happens when I open firefox. I don't get any webpages. I get this message:

Denied Please upgrade your browser. Use the 'Check for Updates...' option on the 'Help' menu to upgrade to the latest version. You will need to restart Firefox when prompted for the update to be applied.

But I am on the latest version and I don't even have a 'Check for Updates' option. Tried all of the above. Still nothing.

If there are problems with updating or with the permissions then easiest is to download the full version and trash the currently installed version to do a clean install of the new version.

Download a new copy of the Firefox program and save the disk image (dmg) file to the desktop

• Firefox 13.0.x: http://www.mozilla.org/en-US/firefox/all.html

• Trash the current Firefox application to do a clean (re-)install
• Install the new version that you have downloaded

Your profile data is stored elsewhere in the Firefox Profile Folder, so you won't lose your bookmarks and other personal data if you uninstall and (re)install Firefox.

• http://kb.mozillazine.org/Profile_folder_-_Firefox

EDIT: See How to download and install Firefox on Mac if you need more help. (I didn't see cor-el's reply before I posted mine)

If trashing Firefox and then installing a fresh new copy of Firefox 13 doesn't help:

Could you open the Help menu, click on "Troubleshooting Information", click the "Copy all to clipboard" button, then paste the contents here in the Reply box? See the article, Use the Troubleshooting Information page to help fix Firefox issues under Copy all to clipboard if you need more help with that..

Also, if possible, could you take a screenshot of the "Denied" message and add it to your reply? See How do I create a screenshot of my problem?. It might also help if you tell us if the message appears in a new tab or a new window (such as a small "popup" window) and the title of the new tab or window.