I had been using Thunderbird version 52.4 under Windows 10 Home x64 (version 1703) with the default profile at C:\Profiles\TB_old . I installed a daily 64 bit version 58 build (20171026030201) in a different directory and it ran the new account wizard when it started. I expected it to have used the default profile. It turns out it had edited the profiles.ini file from [General] StartWithLastProfile=1 [Profile0] Name=default IsRelative=1 Path=Profiles/y8v9hwk9.default [Profile1] Name=Old TB IsRelative=0 Path=C:\Profiles\TB_old Default=1 to [General] StartWithLastProfile=1 [Profile0] Name=default IsRelative=1 Path=Profiles/y8v9hwk9.default [Profile1] Name=Old TB IsRelative=0 Path=C:\Profiles\tmipnqjm.Old TB Default=1 and created a copy of the existing default profile at C:\Profiles\tmipnqjm.Old TB. I assume it failed to use it because there were no double quotes in the modified Path= statement. Reversing the changes to profiles.ini made by daily got both versions to work. 1. The daily build should not have made a copy of the default profile 2. The daily build should not have modified profiles.ini 3. The code that modified profiles.ini doesn't handle embedded spaces in paths correctly. I don't know if that bug could effect somebody creating a profile normally.
Hmm, I'm surprised a profile was copied. TB re-writes profiles.ini all the time, but that should not lead to changed information like was reported here.
Could be a new feature in Firefox (m-c). It didn't saw a randomized string part (tmipnqjm.) in the profile path so it added one. We should look if there was such a change in m-c checkins.