I was struggle with the same problem for a several weeks - all urls from snap apps were opened in different Firefox instances.
Today I finally figured it out - Open some url from snap app. Navigate this instance to `about:profiles` . Make sure that setted as default profile is the same as the one when you open Firefox normally. In my case this was `default-release` profile. Once you set it as default you are done. All links will be opended in your normal Firefox instance.
I was struggle with the same problem for a several weeks - all urls from snap apps were opened in different Firefox instances.
Today I finally figured it out - Open some url from snap app. Navigate this instance to `about:profiles` . Make sure that setted as default profile is the same as the one when you open Firefox normally. In my case this was `default-release` profile. Once you set it as default you are done. All links will be opended in your normal Firefox instance.