Currently on Firefox 124.0.2, the built-in Translations feature is not available when viewing a PDF. This would be very handy to have as it's already quite useful on the web.
Implementing it for full PDF translation will take a while, but soon we will release the selection translation feature which will work in PDFs too (you will be able to select text on a web page or on a PDF and translate it).
I opened this topic precisely to suggest this: it would be fantastic [and the translator implementations are for me, Italian, the biggest problem of Firefox compared to Edge (desktop and for iOS)].
The user interface with which Edge translates parts of PDF is very well done because it opens a sidebar on the right that remains open and automatically translates the text you select