Click Tools in Menu Bar. ... Select Settings. ... Click Privacy & Security located on the left side. Select Block pop-up windows below the Permissions section. ... Exit and relaunch the browser.

Let me if that helped. thanks!

I can't find an option to open tools on the menu bar

Did you read the Firefox help on popups yet?

https://support.mozilla.org/en-US/kb/pop-blocker-settings-exceptions-troubleshooting

Is this a problem with tab preview? When you hover your mouse over a tab up on the tab bar, Firefox shows information in either of two different styles: with a thumbnail image of the tab contents or title and URL only. If you want to turn off thumbnails, there is a checkbox for that on the Settings page.