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Consider the following two sentences, dealing with a singular "ISP":
(1) The virus penetrated the Internet Service Provider's (ISP)'s firewall.
or
(2) The virus penetrated the Internet Service Provider (ISP)'s firewall.
The subtle difference is that (1) has a possessive apostrophe both on the term and its abbreviation, whereas (2) only has it on the abbreviation. Which of the above two is more commonly preferred?
If you can't rephrase, then:
... the Internet Service Provider's (ISP) firewall.
The abbreviation is included in the sentence, rather artificially, solely to inform the reader of it, so make that the sole job it does.
By the same token, if you took the opposite approach to defining abbreviations on first use:
... the ISP's (Internet Service Provider) firewall.
But, rephrased is likely better.
Technically, you don't need the possessive apostrophe on the abbreviation at all. It's an abbreviation; only the first letters in each word are represented.
