Like most materials, glass expands when heated.
Heat breaks glass vessels when the glass is nonuniform,
i.e. has varying densities at various places. If some parts
expand faster than others, this introduces stresses that
may break the vessel. It is generally supposed that
older glass vessels are more uniform than modern
ones, thus can accept more heat without breaking.

Well, not all "glass" is the same.

Pure quartz glass has a very low thermal coefficient of expansion.
Standard "Soda-Lime" glass has a relatively high thermal coefficient of
expansion.

The "Boru-silicate" glass features a thermal coefficient of expansion
close to pure quartz. It's advantage over quartz is a MUCH lower
melting point.

The original American Pyrex (r) was a Boru-silicate glass. Apparently,
the owners of the trademark decided that in the US market they would use
"tempered" soda-lime glass in place of the Boru-silicate glass.

The Tempered glass is good stuff. It's very, very break resistant and
is moderately resistant to thermal shock. (It's actually created via a
"thermal shock" process whereby the hot glass surface is suddenly
cooled; another process uses a "case hardening" process when Potassium
(K) replaces Sodium (Na). The potassium atom is a bit larger ad that
places the surface under stress which has the same effect as quickly
"tempering" the glass.

The neat thing about tempered glass is that when it does break it breaks
into small pieces which are unlikely to cause a fatal injury. OTOH,
true Pyrex (r) will, when broken, generate good sized slithers.

The types of lawsuits generated by broken tempered glass are as in the
noise as compared to that generated by broken soda-lime glass.