In my quick test I couldn't find any bad thing for enabling the content sandbox for Thunderbird.
Attachment #8567517 - Flags: review?(bwinton)
As far as I know, e10s has not been ported to Thunderbird, and sandboxing depends on e10s. Bug 1083344 has rules for the "content process", not the whole app. The above patch (attachment 8567517 [details] [diff] [review]) just changes a setting, but the code to support it is still Firefox only. I'm afraid there is much more work needed to bring sandboxing to Thunderbird.
Oh, I have overlooked that sandboxing depends on e10s. Than I will convert this bug into a general sandboxing for Thunderbird bug.
Summary: Port Bug 1083344 "Tighten rules for Mac OS content process sandbox on 10.9 and 10.10" to Thunderbird → [Mac OS X] Process sandbox for Thunderbird
You'd also need a more general "e10s for THunderbird" bug. Currently none exists. Unless you intended for this bug to do that.
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