The [fɑː] that you heard is an ordinary pronunciation variant of "fire". This kind of variant doesn't only occur in shouted words. In fact, I'd think a "triphthongal" pronunciation [aɪə] (what you probably think of as the "usual" pronunciation of "fire") would be more, not less common in an emphatic setting like that.
What you are hearing is "smoothing," a feature of pronunciation that optionally applies to the sequences /aɪə/ or /aʊə/ in many non-rhotic accents of English. The high glide or semivowel in the middle is elided, and in modern accents the schwa is usually assimilated to the first vowel, resulting in a long monophthong. In modern British accents with smoothing, generally /aɪə/ is simplified to [ɑː] (more or less the same vowel used in the word "father") and /aʊə/ is simplified to [aː] (a vowel that doesn't really occur in other contexts; it's similar to a long version of the vowel in "trap"). Some other slightly different outcomes are possible in more old-fashioned accents.
You can read more detailed descriptions of smoothing on Geoff Lindsey's Speech Talk blog ("Smoothing, then and now"; this has some nice audio files) and John Wells's phonetic blog ("triphthongs, anyone?", "the Cowell jumped over the moon," "our cake").
A slightly different, but related phenomenon: even accents that don't usually have smoothing in content words like "fire" may have it in certain frequent function words. I have a rhotic accent, but I usually have "triphthong" smoothing in our [ɑ˞], and I think I have it at least sometimes in I'll /ɑl/ and while /wɑl/ (but I think when this word is unstressed, I often reduce it even further to [wəl]). These words may also behave differently in accents that do have a generally-applicable process of smoothing; for example, it seems some British English speakers might use [ɑː] for our despite using [aː] for the smoothed pronunciation of power.