When you click on a link to open the link in a browser, the computer uses the default information to select what to do. How your browser opens eg: having focus in your case, is down to the settings you have in your browser.

So this is really a browser issue not a Thunderbird issue.

Suggest you try this: open Firefox

• In top address bar type: about:config
• It will say 'Here be dragons' :) click on 'Accept the risk'

• In top search type: Divert
• look for this line: ‘browser.tabs.loadDivertedInBackground'
• double click on that line to change value from false to true.

• Restart browser.

In Thunderbird click on a link and test to see if link opens in browser, but Thunderbird still has focus.

Thanks for your suggestion. Unfortunately, clicking a link in TB immediately changes focus to FF.

I re-tested the information. I've used it in the past succesfully, but no longer use it.

With Firefox closed and TB open. If Firefox was closed then the first time I click on link in TB, it opened a new Firefox window in focus, but all subsequent clicks on links in Thunderbird opened a new tab in Firefox, but retained focus in TB.

Unfortunately, it also meant that clicking on links in Firefox also opened a new tab, but not in focus which is usually not desired when using the browser. I belive that was the reason I swapped back.

Located a couple of Firefox forum questions as this is really a Firefox issue. Try this info: https://support.mozilla.org/en-US/questions/1066419

Firefox Quantum.New tab gets focused no matter what setting: https://support.mozilla.org/en-US/questions/1177157