This is a list of various difficulties encountered over two months in Thailand using U3 with Avast. And using FF & TB portable versions, not U3 versions because portable had the latest versions available.
1) TBP: Lost the file mail/msgFilterRules.dat 3 times, 14kb reduced to 1kb. Had to copy in from a backup thumb drive.
2) On every public computer need to go into WinExpl/tools/folder options/view and check display contents of system folders & show hidden files. Else cannot view removable disk (I:)/system/apps/ with it's FF & TB portable exe files to start the programs.
At Cathay Pacific Club computers these settings cannot be reached, so could not use a thumb drive several times when there was time to spare sitting at airports.
3) U3 Avast sees prgrm files/goldensoft_b & GoldenSoft as trojan -- win 32:LaSta-B (trj) on several computers in Asia including Cathay Pacific Club.
4) Multitudes of public computers are virus/trojan laden or at least so found by Avast. And shop owners won't/can't clean up -- even in my hotel in BKK w/only 2 computers (1 of which was Win Millenium which wouldn't run the U3)
5) FF & TB are slow start up, as long as 3-1/2 minutes. Close/restart FF takes only 30 seconds, not another 3-1/2 minutes.
6) Cache in FF set for 0mb, yet cache folder was 56mb including cache-001-, -002-, and -003-. Deleted all. These were not all from the original install since dates varied.
7) Bookmark backups were 5 files in addition to the 1 in profile - deleted.
8) Data/profile had 153 BM files. Cookies.txt had 73, session store-x.js had 376, bookmarks 5. Deleted all but 1 of each. TB had multitudes of TmpRules-x.dat. Deleted under both news & mail.
9) Eject U3: Wait several minutes after closing FF,TB & Avast, receive window: "U3 Launchpad - An error has occurred while attempting to eject U3. Please check for any open docs or prgrms running on the device & try again later." Many times would shut down the computer and remove when light is out. Note that windows task mgr shows nothing running except for launchpad.exe which I would stop and still the thumb light remains on. Safely Remove Hardware was still inoperative. So most of the time, simply pulled the plug after 5 minutes. This would be understandable if it occurred a few times, but it is the majority.
10) TB RSS feeds didn't operate - although I didn't try reinstalling on every computer.
11) MailWasherPro which I use to screen spam is not available in portable version. RoboForm requires a separate license for each thumb drive, including backups -- plus the base program on the desktop. (and it operates far better than FF password manager, or for completing forms.
Based on the foregoing experience, unless others have solutions, it is apparent that the thumb drive is not the best method and guess I'll purchase a laptop for the next trip in August.