You will have to middle-click a bookmark to open the bookmark in a new tab.
Otherwise it will always open in a the current tab.

There are a lot of extensions that allow to open a link or bookmark by default in a new tab, so you can do a search on the Add-ons page (Tools > Add-ons > Get Add-ons) if you prefer a simple left-click.

Thank you Cor-el, I have just added an Add-on. Thank you again.

Thank you

Al

You're welcome

I have the opposite problem. My bookmarks always open in a new tab; I already have a number of tabs open, I want one tab for the bookmarks I check from time to time, not a new tab every time. I can't figure out what extension would cause this; is there another option?

What tab configuration are you using and where do you want to open the bookmark?

If a pinned tab has focus then Firefox will open a new tab for links.
If you want links to open in a specific tab then this tab should be selected (have focus) when you click the bookmark.
There are a lot of extensions that can force a new tab by default (left-click), so you wouldn't need to middle-click the link to get a new tab and you can't open in the currently selected tab.

If it works in Safe Mode and in normal mode with all extensions (Tools > Add-ons > Extensions) disabled then try to find which extension is causing it by enabling one extension at a time until the problem reappears.

Close and restart Firefox after each change via "Firefox > Exit" (Windows: Firefox/File > Exit; Mac: Firefox > Quit Firefox; Linux: Firefox/File > Quit)

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