Type about:preferences#content<enter> in the address bar.

On the bottom of the page, find Languages. To its right, press the Choose button. Make sure the correct language and district is selected.

Did you check for issues caused by the used font?

Can you paste the typed and invisible text elsewhere?

You can check for corrupted and duplicate fonts and other font issues:

• http://www.thexlab.com/faqs/multipleappsquit.html - Font Book 2.0 Help: Checking for damaged fonts
• http://www.creativetechs.com/iq/garbled_fonts_troubleshooting_guide.html

The problem also happens in Hebrew posts in Facebook. I inspected one such post and see that the system font used is Lucida Grande. What do I do now?

You could try to remove the Lucida Grande temporarily to see if that helps.

And where do I find the font? Is there a separate folder for Firefox system fonts?

You can use Font Book like I posted further up to mange the fonts and remove it.

What ended up fixing my problem was installing "Theme Font & Size Changer" add-on, then changing the font in the Firefox searchbox through the add-on.