Some of the last major pieces of text work for Gecko 1.9/Firefox 3 are in place. Several days ago I landed a patch that sets text-rendering:optimizeLegibility for text inputs and XUL root elements (by default, it will inherit to the descendants of XUL root elements too). This means we automatically get high-quality mode (ligatures, kerning etc) for text in XUL documents (e.g., XUL application UI) and text inputs.
Today I landed another patch to compute accurate glyph bounds for all text in high-quality mode. This means dynamic changes to such text should redraw correctly even for fonts with glyphs that extend outrageously far outside the font ascent, descent, etc. Here's a demo ... this textarea requests Zapfino and DejaVu Sans; so it will work best on Macs, which have Zapfino preinstalled, but there should also be an "fi" ligature on Linux systems that have a recent version of DejaVu Sans installed. (I'm not sure what fonts to request for Windows, please advise!) Try selecting and editing around a ligature. Of course you will need a Firefox trunk build as of a few hours ago. If you have Zapfino, for a good time try entering "Zapfino" :-). It might not work in feeds or other contexts that sanitize content.
Ideally, we would enable accurate glyph bounds for all text, not just high-quality text, because without those bounds there can be repaint bugs even in "normal" text (e.g., the tail of italic j in many fonts extends beyond the left edge of the character cell). What we're doing here is a compromise so that we don't eat the performance cost of getting glyph extents for all text. (See my Glyph Bounds Problem post for more details.) However, at some point we should do some measurements to see exactly what the performance cost of turning it on always would be.
Similarly, we should also measure the performance cost of turning on high-quality text rendering always.
Update Added Segoe Script to the textarea for Vista users.