my feedback comes on the status bar. I suggest you try starting Thunderbird in safe mode (without add-ons from the help menu) and see if the problems you are having continue. Note that Ubuntu has it's own build and add-ons to integrate unity. So you might also want to ask in an Ubuntu forum.

Matt, Many thanks for your swift and helpful reply. Yesterday, I found the option under tools, to clear everything in browsing history, cookies and cache. It may appear strange that I wasn't aware of this, but my practice is not to open any links directly from Thunderbird, and it never occurred to me that anything significant was resting in there. Also, I have set preferences to clear on closing Thunderbird; which is similar to my settings for Firefox - although Thunderbiird isn't quite as comprehensive. The posted problem hasn't occurred since. I will also use your advice to make sure I have caught all issues. Many thanks :)