[CentOS] clicking on link in Thunderbird fails to open page in Firefox

This worked fine for years until I updated firefox and thunderbird 
recently.  Now, however, when I click on a link in an email in 
thunderbird, rather than open the indicated webpage in firefox, nothing 
at all happens.

network.protocol-handler.app.http in tbird's about:config specifies the 
executable /usr/lib/thunderbird/open-browser.sh and running the latter 
at the CLI with a URL as an arg does work as expected: the spec'd URL is 
loaded into firefox.  So I suspect a bug in tbird.

Anyone else having this problem?