This is strange, perhaps Mac/Linux specific. Do you have menu ≡ -> Translate page...'?

Firefox Translations does not work on older CPUs that don't support SSE 4.1.

I have a CPU which supports SSE 4.1, but I have no Firefox Translations icon and it doesn't exist in the menu.

My thought is that it could be missing because of an extension. However, I have no idea which one and I don't really want to go through 30-40 extensions to figure it out.

Note that you do not get the translation icon for pages that are in the preferred language for displaying pages list, in this case use the "3-bar" Firefox menu button.