Firefox(orange) button > Preferences > Preferences > Content
Block pop-up windows > Exceptions for allowing popup sites

see more

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

Hello eulerhelper, also you can do it from about:permissions in location bar, see : Give certain websites the ability to store passwords, set cookies and more

thank you

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 or otherwise make changes.

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