I have both Thunderbird version 52.9.1 and 60.6.1 installed under Windows 10 Home version 1809, sharing the same profile. I installed the latest beta (67.0b1) using custom setup and it did not find my existing profile. Instead it created a new profile and ran the new account wizard.
I re-tested using the older versions of Thunderbird and they did not have a problem finding and using the original profile. I then edited the shortcut for the beta to specify the original profiles location using command line arguments. It found it and used it. I exited the beta, edited the shortcut to remove the arguments specifying the profiles location and ran the beta again. It didn't use the original profile, and ran the new account wizard again. It is using the same profile (C:\Users\Eric\AppData\Roaming\Thunderbird\Profiles\ak29d4a7.default-beta) that the beta created the first time I ran it.
It appears that the Thunderbird 67 beta is using a dedicated profile per https://support.mozilla.org/en-US/kb/dedicated-profiles-firefox-installation and https://www.mozilla.org/en-US/firefox/dedicated-profiles/
I checked the release notes. There was no mention of this. There should have been.
A typical user doesn't know or care about a change in the Mozilla toolkit, they expect Thunderbrid to continue using the same profile. Many install beta or earlybird builds and expect them to just work. They don't have the mindset of treating the other channels as something separate and/or inherently risky. They just want the latest bug fix and/or enhancements. You can argue whether this is right or wrong, but its a very common attitude.
That is why I didn't set the bug report type to "enhancement". Thunderbird users are used to every channel using the same profile. The Firefox explanation for dedicated profiles is "This will make Firefox more stable when switching between installations on the same computer". Most Thunderbird users don't switch between installations on the same computer after installing another channel. We also don't have an equivalent of Firefox Sync.
I expected Thunderbird to transparently workaround this change in a external library since I was able to get Thunderbird to use the original profile by using command line arguments, and it knows where the profiles.ini file is. It doesn't seem to be a implementation issue, it seems more like a question of policy.
If its decided that its too risky to disable dedicated profiles (due to future changes in the Mozilla toolkit) something like a warning message with a option to clone the default profile as a channel specific profile should have been provided. However, whats direction is decided, the current approach (do nothing) is wrong.
My profiles.ini file contains: