Bugzilla

Sounds like the size of the window for normal use has been altered to a small size. 
This is the sort of question you normally would ask in the Thunderbird Support Forum to see if a simple fix sorts out the issue.
I'm checking to see if this is really a bug or whether you have a file with messed up data.
Sometimes things do not get saved properly if shut down occured prior to file updates or and this is more likely - if another software like Ant-Virus or Cleanup software like CCleaner or Wisecleaner has been allowed to scan profile causing an interference and/or delete session data.
Advise you do not allow those types of products access to profile folders as they can cause havoc.  
Make Thunderbird profile folder exempt from being scanned etc by those types of product.

Try this and report back on results.
In Thunderbird
- Help > Troubleshooting Information
- Under 'Application Basics' - half way down - Profile Folders - click on 'Open Folder'
- A new window opens showing the contents of your Profile name folder.
- Exit Thunderbird now - this is important.
- In profile folder - locate and delete these two files: 'session.json' and 'xulstore.json'
- Start Thunderbird
- those two files will get recreated in profile using defaults, so you may find some things need resetting.

Sounds like the size of the window for normal use has been altered to a small size. 
This is the sort of question you normally would ask in the Thunderbird Support Forum to see if a simple fix sorts out the issue.
I'm checking to see if this is really a bug or whether you have a file with messed up data.
Sometimes things do not get saved properly if shut down occured prior to file updates or and this is more likely - if another software like Anti-Virus or Cleanup software like CCleaner or Wisecleaner has been allowed to scan profile causing an interference and/or delete session data.
Advise you do not allow those types of products access to profile folders as they can cause havoc.  
Make Thunderbird profile folder exempt from being scanned etc by those types of product.

Try this and report back on results.
In Thunderbird
- Help > Troubleshooting Information
- Under 'Application Basics' - half way down - Profile Folders - click on 'Open Folder'
- A new window opens showing the contents of your Profile name folder.
- Exit Thunderbird now - this is important.
- In profile folder - locate and delete these two files: 'session.json' and 'xulstore.json'
- Start Thunderbird
- those two files will get recreated in profile using defaults, so you may find some things need resetting.