The general rule is: monosyllabic adjectives form the comparative with -er, as do duosyllabic adjectives ending in -y.
so: big, bigger; funny, funnier
A few other duosyllabic adjectives can have either -er or 'more x'
e.g. cleverer/more clever
All other adjectives use more, e.g. more modern etc. (source: English Grammar in Use, Murphy, CUP 2012)
However, this appears to be an area of English that is currently changing. Native speakers can frequently be heard using 'more' with shorter adjectives. Less frequent or more emphasised adjectives are more likely to be given the 'more x' form, which suggests that this is the form that is currently productive. Hence 'more tense'.
With 'fierce', I suspect that it could be that the speaker saw it as a two-syllabic word. Or it could simply be part of the same trend.