. . . .I cannot figure out how to replace the current web address without backspacing over the entire address. . . .
Many of Firefox's configuration settings can be found and changed, usually with only one click, by typing "about:config" (without the quotes) in the URL strip and hitting return.
After doing so, paste or type this
browser.urlbar.clickAtEndSelects
into the filter strip and hit return. This will isolate that setting. Double-click it to change it to true.
And with Firefox, if you are more used to highlighting the URL anywhere within it by using a single click instead of three, paste or type this into the filter:
browser.urlbar.clickSelectsAll
and double-click it to change it to true.
This Mozillazine page has a link to the Knowledge Base that has info on the about:config preferences. Click on "Search the Knowledge Base" under "Looking for Help." Then type about:config in the search box.
All the about:config preferences can be changed by creating a user.js file in the same profile folder as the prefs.js file, or by changing the prefs.js file when Firefox isn't running. If it's running when you make changes to the prefs.js file, they won't stick. The Mozillazine site has articles about all of these.
You can also type about plugins (it needs a colon after the word about, but this posting box changes about(colon) to a smiley), and for a chuckle, about:mozilla (there is much history behind about:mozilla scattered around the web).