Just to update an earlier conversation:
Coincidentally, I updated to Windows 11 at exactly the same time of the SeaMonkey 2.53.17 update. I don't know which is the guilty party, but doing so broke my toolbar clock (LCD_Clock_0.4.2). I'll go ahead and make a wild, irresponsible accusation, and just say it was Win 11, since it's GUI alterations seemed to really effect SeaMonkey (new, really thin scroll bar, super rounded corners, blue-underlined field highlights, etc). The clock is still 'there' (I get it's context menu when I right-click), but it's invisible no matter what color I make it.
However, I found this add-on
which works in the same manner (albeit, not graphically. Text-only). Except it displays in the bottom-right of the status bar of the mail window, sliding the "Today Pane" to left. This is non-negotiable, if that annoys you you have to disable the add-on (truth be told, it annoys me a little. I like having the "Today Pane" on the corner. But I need a clock in my browser, so I'll deal). It can be placed anywhere in the toolbar of the browser window, exactly like the lcd clock.