flustered with fonts

[greg wm]

i think i need a "default font lesson" or something.

i'm accustomed to running thunderbird on my X screen (centos 4 or fc5). 
  but when i run it inside Xvnc, it gets a different font, and nothing 
fits in the columns anymore.

i'm accustomed to running gnucash in Xvnc on centos4.  now i've launched
it (still) on my centos 4 box but displaying in an fc5 Xvnc, and here
again it's getting an all new font and nothing fits in the column
widths anymore.

i'm loathe to plunge deep into xrdb and other ancient X lore, expecially
since i suspect from things i've read that there may be several
different generations of font lore that might be coming into play, with
each being documented (or not) in an entirely different place?

there's hopefully a very simple solution like add font specs to some
commandlines or something.  can anyone enlighten me enough so i can know
where to look?

specifically,
what are the fonts thunderbird uses when displaying on my X screen? 
what are the fonts thunderbird uses when displaying in Xvnc?
where may i specify the fonts i want so they become the same?

and similarly,
what are the fonts gnucash uses when displaying in centos4 Xvnc?
what are the fonts gnucash uses when displaying in fc5 Xvnc?
and where may i specify them so they become the same?

tia,
greg wm