I don't see any of my folders.
Solution: Drag slider on left hand side to show folders.
I see my folders, but they're in a different order than before, and I'm not sure if they're all there.
Solution: Use the "All Folders" view rather than "Smart Folders". You can click the left and right arrows at the top of the Folders column on the left-hand side to switch Folder views. "All Folders" is the traditional view.
I don't see the icon that I'm looking for on the toolbar.
Solution: Use the original toolbar instead of the new toolbar. (Go to Help > Migration Assistant and select the option to use Original Toolbar.)
Thunderbird is really slow!
Solution: Check the Google App Status dashboard first to check if there's a known issue with Google mail. If not, have the user unsubscribe from the All Mail folder, which can take up a lot of memory. (Have them click on the account on Thunderbird, i.e. email@example.com in the left-hand column, then select "Manage Folder Subcriptions.) You can also have them try rebuilding the index by right-clicking on a folder, selecting Properties, then choosing "Rebuild Index". If the problem persists over a period of time, we'll need to get their server address and report it to Google.
I have other issues with Thunderbird that weren't addressed here.
Try searching the Thunderbird Support page (external) for the problem. Also, the MozillaZine wiki has a database of frequent Thunderbird issues.