What would make all FireFox Add Ons suddenly disappear?

[macx]

Win 7 Home Premium 64

Setting up computer, installed Firefox and about a dozen mostly security Add Ons.

Everything worked fine (except Text to Voice button would not appear in lower right of FF screen)

Was doing more setup, don't know exactly what was in progress when it happened,
but all of a sudden all the Add Ons except Avast were gone!

Going to reinstall, but would like to avoid that happening again.
Computer is for 84 year old cousin who doesn't know much about computers.

Thanks!

 

[mike1959]

It's possible that some or all of the add-ons you installed were not compatible with the latest version of Firefox.
Or it's possible to start Firefox in safe mode with no add-ons.
e.g. In Firefox, 'Help' menu, then in the menu;' Restart with add-on's disabled'.
But other than that I can't see why your problem would have happened.