moz-extension:// is unsupported.

[RokeJulianLockhart]

I am unable to translate the pages served by "moz-extension://" URIs, such as what and depict: "moz-extension://fd4e202e-e17f-411f-b087-54303670dcbd/data/reader/index.html?id=47&url=https%3A%2F%2Fwww.ukrinform.ua%2Frubric-polytics%2F3550277-es-ta-42-kraini-zaklikali-rosiu-negajno-vivesti-vijska-iz-zaporizkoi-aes.html".

[FilipePS]

An extension only has access to moz-extension:// if this URI belongs to the extension itself. There is no way to access pages with this URI from other extensions.

[RokeJulianLockhart]

@FilipePS, do you know whether a relevant issue exists at bugzilla?

[FilipePS]

I don't know of any relevant issues in bugzilla.
This behavior of Firefox is deliberate and the same with all other browsers.
This is probably for security reasons.

But you can still translate the selected text via a button in the context menu.

An extension only has access to moz-extension:// if this URI belongs to the extension itself. There is no way to access pages with this URI from other extensions.

I think this information is not accurate.
An extension's content script is not injected into any moz-extension:// URL, not even if it belongs to the extension itself.

[RokeJulianLockhart]

"GoogleChromeLabs/link-to-text-fragment#72 (comment)" appears to be relevant, as does "http://bugzilla.mozilla.org/show_bug.cgi?id=1269456#c59".