Hello,

You can use an addon:

• Open Bookmarks in New Tab

For opening bookmarks in a new tab do the following steps -

Bookmarks-->select a bookmark-->Right click-->open link in a new tab

                                        or

Bookmarks-->select a bookmark-->Press Scroll wheel

If you were previously using the Tab Mix Plus extension, that could explain why your bookmarks used to open in a new tab.

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe Mode start window.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Try to open a new window to see if it works there and if it does then close the not working window and close and restart Firefox to see if it keeps working.

I tried that but it didn't work. Thank you though!

Can you open a bookmark or other link in a new tab when you middle-click the bookmark?

That is the way to open a link (including a bookmark) in a new tab, otherwise the link will always be opened in the current tab.
If it was different previously then you had an extension that changed this behavior.

Did you reset Firefox (i.e. have an "Old Firefox Data" folder on the desktop) or do you see disabled extensions in "Firefox/Tools > Add-ons > Extensions"?

Using the wheel click works on the menubar and the location bar, but there is no way to just open a new window with the plus sign that says open a new tab. So what is that button for?

The plus sign on the Tab bar is the New tab button and always opens a new tab.
You can find a new window button the the 3-bar Firefox menu button drop-down list and drag it on a toolbar via Customize if you want to have easier access.
You can also drag the new tab button onto another toolbar or another position on the Tab bar (far left or far right).
Note that you can drag a link on those buttons.

I'm sorry, but the plus sign does not open a new tab either with a left click or a wheel click.

I also tried dragging it to the customized drop-down list. It does nothing there, either.

Do a clean reinstall and delete the Firefox program before (re)installing a fresh copy of the current Firefox release.

• Download the full Firefox installer and save the file to the desktop
  http://www.mozilla.org/en-US/firefox/all.html

If possible uninstall your current Firefox version to cleanup the Windows registry and settings in security software.

• Do NOT remove "personal data" when you uninstall your current Firefox version, because this will remove all profile folders and you lose personal data like bookmarks and passwords including data in profiles created by other Firefox versions.

Remove the Firefox program before installing that newly downloaded copy of the Firefox installer.

• (32 bit Windows) "C:\Program Files\Mozilla Firefox\"
• (64 bit Windows) "C:\Program Files (x86)\Mozilla Firefox\"

• It is important to delete the Firefox program to remove all the files and make sure that there are no problems with files that were leftover after uninstalling.
• http://kb.mozillazine.org/Uninstalling_Firefox

Your bookmarks and other personal data are stored in the Firefox profile folder and won't be affected by an uninstall and (re)install, but make sure that "remove personal data" is NOT selected when you uninstall Firefox.

If you keep having problems then also create a new profile.

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

• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall