NVDA not working with INSERT+F7


I wondered if anyone can help. A colleague is trying to use NVDA on her laptop, but whenever she selects INSERT+F7 to access the Elements list, a slider going from 0 to 100% briefly appears on the left of her screen and the Elements List doesn't activate. She's uninstalled and reinstalled NVDA, but that didn't make any difference. 

I have had a look on the internet, but couldn't see a reason why this is happening.

She's using NVDA 20.2 and Firefox 78.20.

Many thanks

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