Do you see them if you disable hardware acceleration?

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"

Try to toggle some of the Boolean gfx prefs on the about:config page to see if disabling some features helps. Filter: gfx

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

Both suggestions don't seem to be working. I tried restarting FF for each boolean change and disabling the hardware acceleration just to be sure, but the symbols still won't display.

Do you have the Symbola font installed?

Does it help if you disable the system fonts and switch to a different default font?

• Tools > Options > Content : Fonts & Colors > Advanced : [X] "Allow pages to choose their own fonts, instead of my selections above"

• http://www.alanwood.net/unicode/miscellaneous_symbols.html

Firefox 4 is more demanding about fonts and may refuse to use a specific font.

Disabling the system fonts did not show a change.

I did not have the Symbola font installed ... tried installing it, but it only brought up the characters I wasn't able to see before (see my original image in the question above). The symbols that were displayed as squares and numbers now show the actual symbol, but the symbols which did not display are still not showing.

It still sounds to me that you have a font that is either corrupted or that Firefox 4 doesn't like or use for other reasons.

Did you try other fonts? You can also try to install the DejaVu font (also mentioned on the Alan Wood page) and see if setting that font as the default help to identify a font problem.

• http://en.wikipedia.org/wiki/DejaVu_fonts