You can accomplish what you want with a user style. Here are step-by-step directions. If you need an

You can accomplish what you want with a user style. Here are step-by-step directions. If you need any more help, just ask.

Leave "Allow pages to choose their own fonts" enabled.

Install Stylish.

https://addons.mozilla.org/firefox/addon/stylish/

After Stylish is installed, open the Add-ons Manager, User Styles category.

Click the Write New Style button at the top.

In the style editor window, paste either of the following. Give the style a name and then click the Save button.

Variant #1
This is the easiest and safest thing to do. It applies your default sans-serif font to all sites, except those you specify. To specify exceptions, replace the placeholders with actual domain names. To add new exceptions, add another | symbol followed by the domain name.

@-moz-document regexp("https?://(?!(example-of-an-exception.com|www.placeholder.site-exception.net)).*") { * { font-family: sans-serif !important; } }

Variant #2
This requires a bit of work on your part. You must know the font that's being used to display the symbols in question. Right-click a symbol and choose Inspect Element. In the Inspector pane, click the Fonts tab. You should see the font family names listed there, labeled with "remote". The actual font family name is below, under "Used as:".
You're going to want to specify your ideal font first, then the site's fonts after that. If certain characters can't be rendered with the font you want, then Firefox should fall back to the second font in the list, or the third, and so on. If none of the specified fonts contain the required characters, then Firefox will display the Unicode codepoints (rectangles with numbers in them).

@-moz-document url-prefix("http://"), url-prefix("https://") { * { font-family: Arial, "Segoe UI Symbol", FontAwesome, PlaceholderFontOne, "Placeholder Font Two", sans-serif !important; } }