I've been using Firefox forever

Thunderbird?

when sending a message, the "Attachment Panel" is not showing.

Use the 'Attach' toolbar button for the first attachment. This will bring up the panel. Any subsequent attachments can be dragged to the panel directly.

I don't think this behavior has changed recently.

Thank you for this. It does work. However, in the past it has always been there and I didn't need to open the file using the attach toolbar for the Attachment Panel to appear. So now I need to open a file to have this appear. It has always been easier to drag the file there vs having to find it by hitting attachment. So, for me, something has changed but I guess I can live with it. Thanks again.

Did you install an add-on for this?

Maybe I did do an add-on. I found one that said:

Attachment Options 1.0.1 by squib

Allows customization of the appearance of the attachment pane. Not available for Thunderbird 38.5.

This add-on listing is only for users of older versions of Thunderbird; if you have Thunderbird 24.0 or higher, you can already set the attachment pane to be expanded by default: just right-click on the attachment toolbar and check "Initially Show Attachment Pane".

I'm not sure if this was added on or not but again, it was always there as of yesterday and today the panel isn't showing unless I find an attachment first and then it works for that one email. I tried what they said above but "Initially Show Attachment Pane" wasn't to be found. Wish it were there.

You have already spent enough time helping me so I will say thank you again.

This add-on was for the attachment pane when receiving messages with attachments.

You can automatically bring up the attachment panel in a Write window by adding a couple of lines to the file userChrome.css in your profile folder. See http://forums.mozillazine.org/viewtopic.php?f=39&t=1800625