Firefox 4 b5 & b6 : accNavigate(NAVRELATION_EMBEDS) returns 0x80070057?

Hi All,

I have just installed Firefox 4 beta 6 on two of my computers, both
running Windows XP SP3.

On the first one, calling accNavigate(NAVRELATION_EMBEDS) on
IAccessible interface returns S_OK, just like with previous versions.

On the second one, calling accNavigate(NAVRELATION_EMBEDS)
systematically returns 0x80070057.

I am nearly sure my code is not mistaken and I suppose the trouble
comes from a bad installation, because of the behaviour of tools like
Microsoft Inspect32 or MozInspect (a tool Aaron gave me a couples of
years ago) : on the "good" computer, you can reach all fields of html
pages, on the "bad" computer, you just see the top level window, the
system menu and the title bar...

I tried to desinstall, reinstall, reboot, beta 5, beta 6, always the
same bad behaviour on the same computer... I also tried to register/
unregister the accessiblemarshal.dll... but no way.

Does anybody have a clue?

Thanks!

Sylvain