Comment 58 for bug 168261

Yes, it's possible: that's how I was able to do it, and calculate how much it costs us in increased build size. It isn't enabled by default because nobody has presented a convincing reason for it to be, including you, I'm afraid. My understanding of your desired symbol-enabled mail client is that incoming text emails will be translated into SVG symbols (not that I understand why they need to be SVG, rather than images), and that messages will be composed with symbols, then translated to text for sending.

That will be an extension to Thunderbird, and the person who writes that extension will absolutely certainly be capable of building Thunderbird to have an SVG-enabled build to write the extension, and in fact is likely to find it easier to distribute a custom build rather than an extension that can be installed in any SVG-enabled Thunderbird. Just shipping the default build of Thunderbird with SVG enabled isn't going to magically make that extension happen.