Hello TinyCoreTeam,
actually I'm running TinyCore-10.0 with Firefox-66.0.3 (the latest one, over firefox_getLatest)
My preferred mail provider is
www.posteo.de (they use PerfectForwardSecrecy PFS).
When I start posteo.de with my mail address, add a receiver mail address and some text and click the tab to attach a file, then firefox closes immediately all of the firefox windows that are open at same time.
Other programs are not affected.
The download of Mails attachments from posteo.de however works fine. (even with AddOns like eMatrix et al)
With other email providers like
www.freenet.de firefox sends eMails with attachments and no problems.
posteo forbids a list of Win executable file types, but I do not even come so far.
The palemoon browser on TinyCore is a replacement for sending eMails with attachments.
Firefox-66.0.3 on a daily updated Manjaro also allows mail attachments over posteo.de.
To avoid cross reactions I took a naked TinyCore-10.0 installation with all of the actual updates and only downloaded and ran firefox_getLatest to have a fresh firefox-66.0.3. The same result: Open firefox, open posteo.de, click on the [Attach file...] tab,- crash.