Thunderbird hasn't had any major interface changes for the past year or so, so I have a feeling any visual changes you find are due to being setup differently. The Linux and Windows version have always looked almost identical. That is to say, that you may not get what you're looking for by downgrading. But if you are determined to do so, here's a guide. Be sure to back up your current profile before proceeding.

To downgrade Thunderbird:

1. Download your preferred older version of Thunderbird from

<https://releases.mozilla.org/pub/thunderbird/releases/>.

Be mindful to get the proper 32- or 64- bit version for your computer platform.

2. Delete the COMPATIBILITY.INI file from Thunderbird's profile folder:

  a. Open Thunderbird
  b. Open the Troubleshooting page via Menu <Help | Troubleshooting information>
  c. Click Profile Folder -> Open folder
  d. Close Thunderbird
  e. At this point you should make a backup of everything in this folder just in case anything goes wrong
  f. Delete COMPATIBILITY.INI from the folder

3. Use the installer to install the older version, overwriting the current version.

4. Launch the now-downgraded Thunderbird.

We'd much prefer you state the details of your problem, if for nothing else that there is zero guarantee that going back to a previous version will help your situation.

Also, going backward could also create problems you didn't have before.