1. Open the Browser Toolbox, switch to the Accessibility tab. I think this enables the accessibility service or something.

2. Then switch to the Console tab and run:

   gBrowser.addTrustedTab("http://example.com", {createLazyBrowser: true, skipLoad: true})
   

Expected: no warning
Actual: tabbrowser-tabs.js:2099:22 triggers this warning:

Empty string passed to getElementById().

https://searchfox.org/mozilla-central/rev/fc5c4461124b8572442c71bc34947bee68b75551/browser/base/content/tabbrowser-tabs.js#2076-2100

    /**
     * Returns the panel associated with a tab if it has a connected browser
     * and/or it is the selected tab.
     * For background lazy browsers, this will return null.
     */
    getRelatedElement(aTab) {
      if (!aTab) {
        return null;
      }

      // Cannot access gBrowser before it's initialized.
      if (!gBrowser._initialized) {
        return this.tabbox.tabpanels.firstElementChild;
      }

      // If the tab's browser is lazy, we need to `_insertBrowser` in order
      // to have a linkedPanel.  This will also serve to bind the browser
      // and make it ready to use. We only do this if the tab is selected
      // because otherwise, callers might end up unintentionally binding the
      // browser for lazy background tabs.
      if (aTab.selected) {
        gBrowser._insertBrowser(aTab);
      }
      return document.getElementById(aTab.linkedPanel);
    }

Well, of course, if lazy browsers should return null, then this should be done explicitly, not by relying on document.getElementById("").

Set release status flags based on info from the regressing bug 1700708

https://hg.mozilla.org/mozilla-central/rev/9286536a7dbe

The patch landed in nightly and beta is affected.
:Oriol, is this bug important enough to require an uplift?
If not please set status_beta to wontfix.

For more information, please visit auto_nag documentation.

Warning in console doesn't seem important enough.

I managed to reproduce this on Firefox 100.0(20220428192727) on macOS 11. Verified as fixed on Firefox 101.0b7(20220515185854), Nightly 102.0a1(20220515214519) on Win10 64-bits, Ubuntu 20.04 and macOS 11.