Firefox Portable + "normal" thunderbird

Hi all, I'm new here, but I used firefox portable since months ago...

In our company, we use a frozen version of firefox to maintain java and flash.
I've been testing portable firefox (same version, ESR 52.9) for a while, and found one problem when, inside portable firefox, I try to open and eml file...

It's say that Thunderbird is already open, I must close it before open this attachment. Same message when trying to open a second instance of portable Firefox (but this is not the problem). Thunderbird in our computers, is not portable.

If I do the same with a nonportable firefox, it can open the eml file inside thunderbird, even when Thunderbird is still open.

Any tip on this? Is a cross error between portable and non portable app, didn't find nothing on this forum