Controls over tabbed browsing is minimalist, in so far as notepad and ed are minimalist text editors; Last year I tried TabMixPlus on one machine and was annoyed that such a thing could be necessary, just for basic tabbing features. (Konqueror and Opera are much better then extensionless Firefox)
The default theme sucks, and good custom themes that have kept up with releases can be hard to find. On my last hunt, I laughingly ended up with a variant of the 'first' theme I ever setup, all the way back in Firefox 1.0.x !!!
Until recently the interface for managing 'Applications' to open stuff with, was usually useless; by until recently, I mean for a user who has been here sine 2004~2005; and old enough to know that the name and 'logo' has changed somewhat ^_^.
For years plugins have been a major pain; a plugin should never be able to crash the ENTIRE web browser, or worse! Render the entire web browser unresponsive -- the ability to restore the users session is an excellent feature, not a band aid for the problem.
The ability to customize things is ridiculessly low, unless you want to get into themeing or add-ons; take a look at Konqueror, for a starting point on keybinding lol. I'm not even going to speak of Firefoxes interactive-scriptability...
On my Windows NT 5.1 machine, tested under Firefox v3.0.3, v3.0.6, and 3.0.9 tested; have never experienced this under BSD or GNU/Linux systems; but under Win32 the following also....
Images are about 3 times the size; if you go by dimensions in terms of window.resizeTo()'ing the browser window to match the on-screen size of the image.
Text is huge; I had to turn fonts down to size 10; SeaMonkey runs at 16 on the same machine, and the FreeBSD box uses the defaults.
Firefox initially refused to remember my home page in the preferences
Firefox blatantly refuses to accept 'Show my home page' on startup, and always 'starts up' with the 'Show my windows and tabs from last time' option. EDIT: seems to be doing it on unix as well; piece of shit web browser!!!
Except for the last three, I'd reckon the problems are because my Windows machine does not use the default DPI setting in Windows -- if that causes a problem, I would consider that to be Mozilla's problem and not Microshaft's ;-).
Note: Over the past 10 years, it has not been uncommon for me to spend that many hours a day, surfing the World Wide Web; so I am not a total moron.