Move PST email to Thunderbird
Thunderbird does not read PST files on its own. To move email from a PST into Thunderbird, export the PST to MBOX with MailRelic, then import the .mbox files into Thunderbird.
Step 1: export the PST to MBOX
Open the PST in MailRelic and choose Export all emails in the archive summary, or Export folder… for a single folder. In the Export dialog, choose MBOX and pick an empty destination folder. MailRelic writes one .mbox file per folder, in the same folder structure as the PST. Details are on PST to MBOX.
Step 2: import the .mbox files into Thunderbird
The version of Thunderbird used in testing did not offer a way to import a standalone .mbox file from its built-in import tool. The ImportExportTools NG add-on for Thunderbird does: install it, then use its MBOX import to add the files MailRelic created, for example into Local Folders.
Thunderbird and its add-ons change over time. If your version offers MBOX import without an add-on, you can use that instead.
What was tested
An MBOX file exported by MailRelic from a PST created by Outlook was imported into Thunderbird with ImportExportTools NG. All its messages arrived, and HTML messages with inline images displayed correctly.
Only email is useful in Thunderbird this way. Contacts and calendar items from the PST are not converted into Thunderbird's address book or calendar.