Leading Rishonim take diametrically opposed positions regarding the status of glass utensils.
According to Rabbeinu Tam, Rosh, Rashba, Ran, and many others, glass is smooth and hard, and even if it had been used to hold boiling hot foods, it does not absorb their tastes. Therefore, even if glass utensils were used with boiling-hot ĥametz throughout the year, they may be rinsed thoroughly and used on Pesaĥ. This is the position adopted in SA 451:26. Similarly, glass utensils may be used alternately for hot meat and hot dairy foods, as long as they are cleaned well in between.
According to Smag, Smak, Mordechai, Agur, and Terumat Ha-deshen, glass, because it is made of sand, has the status of earthenware, which is also made of sand and which absorbs tastes but does not release them. Therefore, if one uses glass utensils with hot ĥametz foods, there is no way to kosher them for Pesaĥ. This is the position adopted by Rema.
There is also a third position, which maintains that glass utensils have the same status as metal utensils, which can absorb and release when boiling hot. However, since glass utensils are fragile and liable to break during hagala, there is no way to kosher them for Pesaĥ (Or Zaru’a, Shibolei Ha-leket).
In practice, many Sephardim follow Shulĥan Arukh’s opinion that glass utensils do not absorb and thus may be rinsed thoroughly and used on Pesaĥ. However, there are important Sephardic authorities who are stringent in this regard, Ben Ish Ĥai among them (Year One, Tzav 14).
In Ashkenazic communities, some follow Rema’s stringent ruling while others adopt the middle opinion, allowing glass to be koshered via hagala on the grounds that today’s glass is stronger than in the past and capable of enduring hagala. There is even more of a tendency to be lenient with tempered glass cookware (like Pyrex), which is fire resistant. Some rule that hagala should be performed three times on glassware.
In practice, Ashkenazic Jews do not kosher glass utensils for Pesaĥ through hagala, in keeping with Rema, and only under pressing circumstances rely upon the lenient position. With regard to prohibitions other than ĥametz, the tendency is to rely upon the lenient poskim, waiting twenty-four hours from when the utensil last contained hot food and performing hagala three times.