I find that when I accidentally close a tab (usually by inadvertently using the swipe gesture, which I'd also like an option to disable, but that's another suggestion ticket), the option to easily undo the error stays available for far too little time. By the time I can even comprehend that I've made an error, never mind make a decision to do something about it and move my finger to tap the button, >90% of the time the button has already disappeared.
I can only imagine that this is even more frustrating for people with slow reflexes and/or fine motor control issues that make interacting with the "undo" button in time a physical nigh-impossibility (doubly so because I can't imagine they'd have an easier time than me with not making the wrong gesture and thus inadvertently triggering this behaviour).
I propose that in addition to the default ~2 seconds, there be options for 5, 10, and 30 seconds, as well as an option to leave the dialogue up until the next time a "significant action" is performed. A "significant action" would include such things as closing another tab and press-hold-moving a tab (but not opening the 3 dot menu or a new tab (important because the 'open new tab' "+" button is right next to the "undo" button)), all of which are things I'm pretty sure the existing dialog already accounts for but which I haven't tested.