Assuming an add-on isn't getting in your way, try this:

1. Click on the Download Manager icon in the toolbar (the down-pointing arrow) to open the Download Manager Dropdown.
2. Find the download you want to pause
3. Right click on the downloading item and select Pause.

It's the same in the pop-out window.. just right click on the item and select Pause

But the pause option is not working. give me any other ideas

Are you using a download Add-on? Have you tried the step described here? https://support.mozilla.org/en-US/questions/981703

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