Searched and didn't find anything on this....
I'm not sure if this is a Thunderbird issue or a ThunderbirdPortable issue, but the little twirling dealie (that's a technical term) that spins when Thunderbird is thinking (in the upper right hand corner of the window) never stops spinning after you check your mail once. The progress bar at the bottom which displays how far along you are with checking your mail also never disappears and shows 0%.
Not sure what else to say about the problem. Let me know if you need more info.