If you place the add-on in Firefox's overflow menu section, opening it renders the UI all wrong. The window is not big enough.
This is how this looks:

Workaround for now: use the new "Open session list in new tab" option.
I'm getting the same issue (Firefox 69 on Ubuntu). Increasing the width in the preferences (e.g. #372) does not seem to work in this mode.
Note: the same issue was present in TSM versions <5.0
This is due to a bug in Firefox.
https://bugzilla.mozilla.org/show_bug.cgi?id=1373490
Many other extensions are also affected by this bug.
I added an option to adjust the size to fit the overflow menu.


Works very well for me, thanks.
Maybe it's my screen size but I'm getting this tiny issue with the "No sessions selected." label being too close to the border with the default sidebar width. It looks much better with a 280 sidebar width.

Hmm, somehow still doesn't work for me:


If you can work around it - great. If not - guess we all have to vote on the Firefox-Bug ...
OK, follow up. It works correctly if I ALSO adapt the "normal" width from the 600 shown in my screenshot to the 348 shown in the screenshot of @sienori . Changing back to 600 (that was the default for me) shows the bug again. Setting both width to 348 shows the following behaviour:

Could it be that one of the widths just influences the left part?
Someone on bugzilla proposed to solve this issue like chrome did:
When the button is clicked, being inside the overflow menu, it gets temporarily moved into the toolbar to show the full-sized window.
Wouldn't this be a good workaround for such an extensive bug on mozillas side?
I mean this will not be fixed soon, it will take (and took already) a looong time.
The solution is to change the entire popup to only show the selected session, with a back button to return to the session list. It should not try to show both side-by-side, and adjusting the widths to squeeze them both in produces an extremely poor experience.
Most helpful comment
This is due to a bug in Firefox.
https://bugzilla.mozilla.org/show_bug.cgi?id=1373490
Many other extensions are also affected by this bug.