Font Issues Fira Sans

You can contact the font developer via Google Fonts and see if they have a fix.

If you installed from Google Fonts, the issue can't be (by definition) Adobe fonts. (You should be careful, not to install both ways, however). 

Is the document with missing fonts something you created or did someone else create it on a different computer?

Several factors can cause missing font issues even when fonts of the same name are installed in a computer. The fonts in the original document could be in a different format (TrueType, Type1 or OTF). The font's build version could be different; fonts get updated to fix bugs or add new features. If someone builds a document using fonts that have been converted from one format to another those font files will be unique. The fonts will have to be included with the document when shared to another computer.

I've seen issues with fonts disappearing from the fonts menu in Adobe Illustrator when CorelDRAW and its font management app is running at the same time. When I closed both applications and then re-launched Illustrator all the fonts in the fonts menu would be restored.