User Agent: Mozilla/5.0 (Windows NT 6.3; Win64; x64; rv:88.0) Gecko/20100101 Firefox/88.0

Steps to reproduce:

I change variable-parameter mail.forward_subject_prefix to UTF8 string
and early versions encoded this OK...
Last 2-3 versions show this value wrong...

Actual results:

When message is forwarded, string was wrong showed...
i.e.
word: "Проследи" -> "ÐÑоÑледи"

Expected results:

word: "Проследи" -> "Проследи" before ":" in subject line of forwarded message...

From https://thunderbird.topicbox.com/groups/developers/T0e8f17a4e0d049c0-Mfdbb318429360c38849cb05f

The code around mail.forward_subject_prefix looks strange:
https://searchfox.org/comm-central/rev/7ed718c90a58141a74da3acbf3f97ad8854e5561/mailnews/compose/src/nsMsgCompose.cpp#1944
Since that's retrieved from a pref, it should be treated as Unicode (UTF-16) and not converted from UTF-8 to UTF-16.

Comment #1 is wrong. I did a bit of debugging. This went broken in TB 78 where Enigmail was integrated into Thunderbird.

Set the preference mail.openpgp.enable to false, and you will see that your prefix will work, I tried with Проследи. Works for both inline forward and forward as attachment. Looks like the processing here is wrong:
https://searchfox.org/comm-central/rev/7ed718c90a58141a74da3acbf3f97ad8854e5561/mail/extensions/openpgp/content/ui/enigmailMsgComposeOverlay.js#501-567

I test this with Thunderbird 89.0b1 (64-bit)...

And here dont work...

Forward works for me in an en-US version of TB 89b1 (x64), but only if I switch preference mail.openpgp.enable to false. Have you done that?

This version dont work with changes of this parameter/value... (mail.openpgp.enable set to false)

Maybe, there is one more side effect of some other variable...

Maybe I dont restarted client...
Now WORK!!!
Thanks!

Today test... OK.

Thanks!

Turning off OpenPGP may be a workaround, but unfortunately not a solution.

getCharPref was used on a value that may be utf8.

Pushed by thunderbird@calypsoblue.org:
https://hg.mozilla.org/comm-central/rev/e8b3ecd00ddf
Use getStringPref() to retrieve mail.forward_subject_prefix when openpgp enabled. r=kaie

Please get into the habit to set the Milestone and the status flags (or else, you will not realise that the bug needs uplift).

Comment on attachment 9231972 [details]
Bug 1706918 - Use getStringPref() to retrieve mail.forward_subject_prefix when openpgp enabled. r=kaie

[Triage Comment]
Small fix for reading a preference correctly, taking for 91b4.

Thunderbird 91.0b4:
https://hg.mozilla.org/releases/comm-beta/rev/e78e7d3c675d