I'm not sure about Ubuntu, but I think Ctrl+Tab should work regardless of the OS. You can also try Ctrl+PgUp/PgDown, they should also work, according to this article: https://support.mozilla.org/en-US/kb/keyboard-shortcuts#w_moving-around-thunderbird

An other option to switch between Tabs is to use Ctrl+<num key> to switch to tab <num>, where the left-most tab (with the folders) is tab 1 and the right-most tab can always be used with Ctrl+9.

thanks Onno but not working - maybe let me try explaining it better. If i have the mail and calendar open, ctrl+tab works and i can switch between them. however, in the mail section, if i have 3 mails open, I am not able to switch between them (maybe the term to be used here is windows!, rather than tab!!)

I'm still not sure what you mean. Do you open each mail in a tab or in a window? If you open them in a tab, the keyboard shortcuts should work as described in the document. If you open them in a window, then you'll need the keyboard shortcuts as defined by your operating system to switch windows. For Windows, this is Alt+Tab, but I don't know the key for Mac/Linux. I think Mac has separate keys for switching between applications and switching between windows from applications, but I don't know which shortcut is used in either case.

From Googling I learned that switching between windows belonging to the same app, you should use [Alt]+[Key Above Tab], so depending on your keyboard layout this could be Alt+` or Alt+§. Anyway, this probably also depends on which Window manager you use in Ubuntu.

awesome Onno - the suggestion - [Alt]+ ` - its also the key above [tab] ... I kept trying so many combinations. am using a Dell 9360 and though Dell seems to be the most compatible with Ubuntu, lots of small issues. Am going to try Mint next week and see whether it makes life simpler