I disagree strongly with what has been said about herbal substitutions.
If you, as a person, were left out of a group, who could substitute for you? Not just anyone, for sure. Herbs are the same thing energetically as people, each very different with a wide variety of uses, and in a forumla as righteous as Fiery Wall, each herb is going to be a heavy-hitter to help you toward your goal.
Herbs are not just used symbolically in magic - they have inherent powers all their own. In many instances in hoodoo and other folk magics, herbs (and prayers) do ALL the work. You are using an old-timey hoodoo formula. These herbs have worked together as a team for hundreds of years. Your success is piggy-backing on this marriage-relationship of botanical helpers.
Typically, hoodoo formulas are not shared. Practitioners work hard to craft their own recipes and ususally only share them within their families. I'm curious as to where you got a recipe for Fiery Wall. If it's from Hermal Slater, it's total garbage, just for the record.
That being said, i would not substitute Rosemary unless you are using this to protect a woman. Rosemary is a protector of women and their children., amongst other traits It's certainly NOT appropriate for a general substitute. Personally, i would add another protection herb: Dragon's Blood resin, Rue, Devil's Shoestring, Angelica Root, Salt, etc. or just leave it out of the formula entirely (many hoodoo practitioners use an odd number of ingredients in their recipes). In my opinion, it's better to leave an herb out entirely than to invite some unknown substitute to the party.
I personally would use Dragon's Blood or Angelica Root because they have a similar high, holy vibration that Frankincense has. Dragon's Blood is also responsible for making the formula red, so it should already be in your recipe. You can find Angelica Root in tea bags at natural food stores. It also goes by its Chinese name: Dong Quai. You can also find Frankincense at Middle Eastern/Greek/Lebanese grocery stores.