Keeping safe version of application and prevent update

Hello to all!
I have problem concerning Firefox. I have masked that package,so there is no update for it. I want to preserve stable version of FF.
It worked for few days,but today,update is offering me to build older version from the version that I have.
I have 38.6.0,update is offering 38.5.0.

Please help.

Thank you!

Try to mask updates.

echo '=www-client/firefox-38.6.0' > /etc/portage/package.mask/custom

There is one problem. Stable version of firefox 38.6.1 is masked. Why ?

Please show the content of all files in /etc/portage/package.mask.