getfirebug.com has Firebug 1.6a20. All tests pass on Firefox 3.6.9. We plan 1.6b1 for early next week in conjunction with Firefox 4.0b4 which looks like it will work for everything except AJAX debugging.
This release includes a preference you can set via
about:config to disable the popup completion dialog,
extensions.firebug.commandLineShowCompleterPopup. We will disable the popup on Linux by default for Firefox 3.6.The Mozilla platform feature we use (panel) does not work correctly on Linux. We will evaluate it on Firefox 4.0. We may add a user interface control for this option if it seems like the completion works well enough without the popup.