'check for updates' in FF3

If you go to Edit, Preferences, Advanced, Update, you will see that you have set this to happen automatically.
That is why it is greyed out.

I think ubuntu only allows updates to firefox through synaptic, and if you manually install a different version of firefox than the one provided with and updated by firefox, I'm not sure if you can then search for updates or have to reinstall from scratch each time to apply them. Ubuntu appears to lag behind on firefox updates, which I assume is to assure compatibility.

it is grayed out in 'Edit, Preferences, Advanced, Update' also, but it is checked.

The current version of FF is 3.0.4 (2008). In Windows, any application will announce an update at any time whereas in Ubuntu, all updates are controlled centrally and will only be released when there is not going to be any conflict between the update and any existing programmes.
When an update becomes available for FF, the "Update Manager" will automatically let you know.

Although there is 3.0.7 version is available, the "Check for Updates" on my FF 3.0.5 still seems grayed out. What should I do? Thanks

@ Honore you need to simply keep your Ubuntu system updated to get the latest release of Firefox

But also please don't append a new question on a already answered or marked as solved question.
Please make new question from here: https://answers.launchpad.net/ubuntu/+addquestion
you will get better chance to get right answer on a fresh tagged "open" question.
Solved or answered questions are usually not read from answering people.

Thank you