Unwanted menu opening

[tDeContes]

• I start to write a message.
• I change apps with the Alt+tab keyboard shortcut.
• I'm coming back to Thunderbird.
• When I start typing again, instead of writing the text, it opens the menus (with the correspondence between the underlined letters and what I type on the keyboard).
  If I don't notice it quickly enough (because I'm looking at my keyboard instead of the screen), it might do things I don't want.

Do you know what could happen at this time?
I'm not sure whether the source of the pb is Text-Insert or Thunderbird.