If TB runs properly when Windows is in safe mode, it most likely means that some program that runs at startup in Windows normal mode is causing the problem. Safe mode bypasses startup programs, and the most likely one to interfere with TB is a security/antivirus program. If you have such a program, make sure that it is not scanning the TB profile folder or secure (SSL) connections. In some cases, this requires that email scanning be disabled or the TB profile folder is added as an exception or exclusion.

Also, it's widely recommended that you exclude CCleaner from having any effect on TB.

Thank you for responding so promptly. I have run msconfig and turned off all the startup programs but in spite of this, TB still fails to start in normal mode.

I have compared the lists of processes in task manager between safe and normal and although I turned off all the items in start up there is a much longer list in the normal view. Unfortunately I don't know what many of them are, or how to remove them. I am enclosing a screen shot of the 2 versions in the hope that you might recognise the offending process and can advise me how to remove it.

The item that may be the cause is indicated by the Bitdefender agent. I would recommend you add the TB profile folder as an exclusion in BD and see if TB runs OK in Windows normal mode. The default location of the profile on Windows is:

C:\Users\<winusername>\AppData\Roaming\Thunderbird\Profiles\<8charstring>.default

AppData is a hidden folder, by default.

RapportService is also a security program promoted by banks, but I don't think it has any effect on email.

Safe mode also loads basic drivers for e.g. graphics and touchpad, but I doubt yours have changed for years. Bitdefender is the first thing to look at.

Thank you again for your help. However, I am a little confused by your advice to exclude the profile folder in BD and then seeing if TB runs OK in Windows safe mode, as TB already runs in Windows safe mode.

I should mention that I have 2 users set up on the Win 7 laptop and am experiencing identical problems (ie unable to run TB or see profile folders) with both users. I also have a Win 10 laptop (one user only) on which TB is OK However, I hadn’t realised that although Explorer was set to show hidden files on the Win 10 system, it wasn’t on the Win 7 one. Having changed that, I can now see the profile folders and have added the profile folder for one user to BD exclusions (leaving the other user’s unexcluded for a control). This does not appear to have had any effect. I have now used MSCONFIG to change the start up to Diagnostic. I was unable to get rid of Rapport as it was not in listed programs and Chrome (where it is probably an add on) would not start. Re-booted - TB was able to start, but system was unable to connect to router and network trouble shooter would not work. Also display was very dim. Re-booted. - No change, dim display, no wifi but TB started OK. Used msconfig to select normal start and re-booted. WiFi connected. After some delay (and several attempts which had to be cancelled) TB started (both users). However, Dropbox and BD are not working, but the original problem seems to have been resolved somehow. Thank you.

Yes, meant to say exclude the profile folder in BD and run in Windows normal mode.

I don't know about why Dropbox isn't working, but adding an exclusion to BD shouldn't affect its real-time background scanning option, which provides the necessary protection to all processes, without the negative effects of scanning the TB profile.

I have a Win7 computer with MS Security Essentials as the sole security program, and TB runs fine with the profile folder added as an exception in MS SE.