I get the following error.
Firefox: 70.0.1
Extension: 2.1.2 (Last updated 2019-11-19)
JabRef: 5.0.0 (2019-11-19)
OS: Linux Mint 18.1

stderr output from native app org.jabref.jabref: File "/opt/jabref/lib/jabrefHost.py", line 34
stderr output from native app org.jabref.jabref: logging.info(f"Got length: {message_length} bytes to be read")
stderr output from native app org.jabref.jabref: ^
stderr output from native app org.jabref.jabref: SyntaxError: invalid syntax
stderr output from native app org.jabref.jabref: Error in sys.excepthook:
stderr output from native app org.jabref.jabref: Traceback (most recent call last):
stderr output from native app org.jabref.jabref: File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 145, in apport_excepthook
stderr output from native app org.jabref.jabref: os.O_WRONLY | os.O_CREAT | os.O_EXCL, 0o640), 'wb') as f:
stderr output from native app org.jabref.jabref: FileNotFoundError: [Errno 2] No such file or directory: '/var/crash/_opt_jabref_lib_jabrefHost.py.1001.crash'
stderr output from native app org.jabref.jabref:
stderr output from native app org.jabref.jabref: Original exception was:
stderr output from native app org.jabref.jabref: File "/opt/jabref/lib/jabrefHost.py", line 34
stderr output from native app org.jabref.jabref: logging.info(f"Got length: {message_length} bytes to be read")
stderr output from native app org.jabref.jabref: ^
stderr output from native app org.jabref.jabref: SyntaxError: invalid syntax