As wrote in the title, when I edit  something rezzed InWorld (no matter if it's a prim or a mesh) my avatar 'kindly' rotate itself to face the object. Most of the times it doesn't matter, but it's quite annoying when I try to 'align' an object to the avatar (ie for comparing the height and the witdh).

So my question is if it's possible to disable in firestorm (or firestorm-like) viewer that behaviour (something to check/uncheck in preferences, or in setting or else)?

Thank you in advance.

Yes you can disable that feature in the preferences section. I think it’s located in the view section.

I'm new so i gfuess this works like this?

It is in move & view in the preferences, look for a thing called "Automatically pose avatar during (select one or more) then deselect "Build/Edit".
Disabling selection targets might also be useful, it is located in "Privacy" under "Lookat".