My guess - and it's just a guess - is that you may be responding above the quote and your sig is appearing below the quote and it's not appearing where you are anticipating it. Anyway, that's a common issue. Let me know and we can go from there. Thanks.

You are 100% correct! How do I get it under my new message??

If you go into the Account Settings for the account in question and look at the Composition & Addressing section there is a drop-down to tell where to put the signature.

Select the below my reply (above the quote) option.

This is with version 128.10 anyway.

THANK YOU!

This probably is a useless place for it, but I'd like to chime-in with my opinion:

• It would be nice to be able to change this setting globally (for all accounts at once as opposed to per-account) and through the GUI. It can be done via config editor (settings > general > config editor). Just set mail.identity.default.sig_bottom = false.
• Default behavior should be to have the email signature below my reply (above the "quote"). Not just because it's Outlook's default and people are used to it. Common behavior among the emailing general public is for serveral back-and-forth replies to stem from an original email, and in this case a stack of email signatures at the bottom of the email thread is meaningless and silly. A reply with signature should be contiguous; previous messages (the "quote" (I consider "quote" to be somewhat of a misnomer)) are there for reference below the reply and separated from it. Note, I don't like to trust email clients' functionality for automatically maintaining an email thread as such; I like having the thread built into each reply.