Yes, of course, there are solutions for PST to MBOX conversion. in fact, there are three simple and effective ways.
1. Outlook PST file to Thunderbird on single Machine.
2. By Google Take Out.
3. By Gmail Synchronization
As you said, you want to import those mails to your Thunderbird account.So you can follow these steps.
Export Outlook PST file to Thunderbird on single Machine. Step 1.) Click on the Start button, then Go to Default Programs, then set Outlook your default program. By clicking on the Set this program as default.
Step 2.) Run Thunderbird and click on theTools then click Import/Export Menu. Step 3.) Select the radio button which appears next toMail then click ‘Next’. Step 4.) Select Outlook from the provided list and click 'Next’.
Step 5.) Click on Finish to complete the import process.
I have described you only one method here, for other two methods you can read an article on the internet, you need to just google this "The Top 3 Ways to Export Outlook Emails to Thunderbird"