Does nothing in Firefox 113

[iav]

Does nothing in Firefox 113, Windows x64.

[vsDizzy]

Thanks for reporting.

[horia-m]

it is still the same situation in dec. 2025, in firefox 146.

[vsDizzy]

I am afraid It won't fix.
chrome.pageCapture.saveAsMHTML API is not supported in Firefox. This is a Chrome-specific API, and Firefox lacks native support for creating MHTML files using WebExtensions.