browser.tabs.executeScript fails in Thunderbird 75.0b2

A private MailExtension add-on that works in Thunderbird 68.6.0 (Windows) fails in Thunderbird 75.0b2 (Windows).

The add-on adds a button to the toolbar that, when clicked, opens a new tab and loads a template html page into the tab and then executes a script file to populate the template with data. This is the relevant code in background.js:

const tab = await browser.tabs.create({
    url: "list.html"
browser.tabs.executeScript(, { file: "populate.js" });

While in Thunderbird 68.6.0 everything works as expected, in Thunderbird 75.0b2 the new tab is created and shows list.html, but no data is loaded into the page. So I assume that populate.js simply doesn’t get executed. Is this a permissions issue? The console shows no error.