Well, I responded to your original post last night but wrote it in an editor window so that I would not lose it if it did not appear on the forum.
Then, I fell asleep before sending it.
I see that you have found a solution. But I'll send what I wrote anyway, it may help with some more clues.
I believe that this situation will be affected by choice of font and size.
So, for those of us who use a very large font it can be a real problem.
I used to have an add-on that allowed me to adjust this on the fly using the scroll wheel. But, it does not work with the latest version of Thunderbird.
Also, if this happens a lot, try changing the default layout to "Wide View"
You do this in view > layout.
That will give you a content pane that is the full width of your display.
Also, this can be affected by the format of the document that you are reading.
In View > Message Body As, you can change Thunderbird to show a letter as Plain Text and sometimes get one that wants to disappear off the right side of the screen to wrap and display so that you can read it.
Most of the time I have mine set on Simple HTML which makes text come out as expected but gets rid of a lot of the other HTML crap that you don't want.
This can come in very handy on some forums that display oddly due to people posting in HTML complete with advertisements.
In general, the text on your content pane will wrap as you decrease the width of the pane until you reach a certain built in minimum. On both my Windows and PCLOS installs with the display set at 1024x 768, that happens at anything less than a 480 pixel width for the content window.
Now, as for the menu bars etc; because you can customize the menus, you just have to rearrange and re-size things on the menu bar until everything that you want to show will fit in your preferred screen width.
The top bar that has the window control boxes on the right side will always automatically change position to match the window size that you drag to. Therefore, you do not have to worry about making things too small and losing the ability to recover.
Also, this is affected by the theme.
The menu bars can be customized to use smaller icons etc. and or to put the labels under the icons rather than beside them.
You do this by right clicking on a menu bar and selecting custom.
Different releases of Tbird have set different defaults for these things. So, that may be why you are seeing a difference between machines.