Security researcher Holger Fuhrmannek used the used the Address Sanitizer tool to discover an out-of-bounds read issue with Web Audio when interacting with custom waveforms with invalid values. This results in a crash and could allow for the reading of random memory which may contain sensitive data, or of memory addresses that could be used in combination with another bug.
In general this flaw cannot be exploited through email in the Thunderbird product because scripting is disabled, but is potentially a risk in browser or browser-like contexts.