First, There is no ticket system for Thunderbird. You probably filed for support on the site for fee-paid services, which does not include Thunderbird. Second, I suggest you just reinstall thunderbird to stabilize where you are and then report issues. (Note: I do not understand the 32-bit x64 comment.)

1) The ticket 150321 was allocated in an e-mail sent by Mozilla Support <product-help@firefox.com> who I suppose posted my problem here. 2) The description of my operating system as "32-bit Operating System, x64-based processor" came from Control Panel - System. I use a AMD FX-6300 six core processor which could operate a 64 bit operating system, but I installed Windows 10 32-bit in order to support some legacy software and hardware. Thunderbird used to operate satisfactorily until recently. In addition to the blank drop-down menus, I am also plagued by "You haven't set up a chat account" with its inoperative buttons.

I have an old laptop with Thunderbird 6 installed which I may have to resort to. I re-installed Thunderbird 115 yet again, but this gave only partial short-lived relief. Thunderbird is quite slow to load alongside these present troubles.

Is there legacy software that Thunderbird is interacting with? I mention that, since that is why you are using the 32-bit version. I also suggest checking what software might be interfering with Thunderbird; often, that is caused by well-meaning antivirus products. What you're experiencing doesn't fit in normal problem areas, and that is why I'm thinking something else is happening here, since you state that Thunderbird works, and then it doesn't.

1) The ticket 150321 was allocated in an e-mail sent by Mozilla Support <product-help@firefox.com> who I suppose posted my problem here.

Sounds like you posted your question in one of Mozilla's subscription products sections and not in the Thunderbird support forum section where this thread is in. To post a thread in Thunderbird support you use https://support.mozilla.org/en-US/questions/new/thunderbird/form

The products Mozilla VPN, Firefox Private Network, Firefox Relay Premium, MDN Plus, Hubs, Mozilla Monitor Plus on https://support.mozilla.org/en-US/questions/new does use a ticket system as they are subscription based and are not meant for Thunderbird support.