As a general rule, once a textbox has focus, usually because you used Orca's e navigation hotkey to jump to it and switched to focus mode or tabbed to it from an adjacent form element, you should be able to just type into it.
That said, I know console applications don't always use standard keyboard shortcuts that are near universal across both Windows and Linux when it comes to editing text, so here are some basic keyboard shortcuts when in a textbox that should also apply to most GUI text editors, word processors, etc.:
Textboxes in web forms come in two versions: single line and multi-line, and best I can tell, Orca doesn't make a distinction between them. In either case, left/right arrow will move the insertion point one character at a time, and ctrL+left/right arrow will move the insertion point to the next whitespace or punctuation character, and home/end to the beginning/end of the line. for Single line text entries, up/down arrow act like home/end, while for multi line, they move by line. PageUp/PageDown will move by multiple lines in a multi-line textbox, but they are rather unpredictable.
Hold shift and use arrow/navigation keys to select text, everything acting like it does for moving the insertion point, including the ctrl+left/right arrow to go by word/string.
ctrl+a: Select all. If a textbox is focused, this will select the contents of the text box. If you are in browse mode, it will select everything on the page.
delete/backspace: will delete whatever is selected, if anything, if nothing is selected, they just delete the next/previous character relative to the insertion point.
ctrl+c: copy selection
ctrl+x: cut selection
ctrl+v: paste last thing copy/pasted
ctrl+shift+v: This isn't useful when editing text in firefox, but if you need to copy something from Firefox or another GUI application into a terminal window, the normal ctrl+v to paste often won't work and you'll need to use ctrl+shift+v to paste into the terminal.
Firefox has a built-in spell checker and Orca should announce an unrecognized word when you press space at the end of the word or move the insertion point into it, and Firefox's suggestions for correcting the unrecognized word are part of the context menu when a textbox has focus.
As for copying the contents of a file into a textbox,, if you type:
file:///path/to/directory
into Firefox's address bar(accessed with ctrl+l), you can navigate local directories, and if it's a file Firefox can read, you can open local files, though Firefox will only open plain text files that have the .txt extension as far as I know. I have several local directories where I'm likely to want to copy the contents of a text file contained therein into a web form, and when I want to do so, I open the appropriate bookmark to a local directory in a new tab, open the file I want to copy, elect all, copy, switch tabs, and paste. Firefox also has an open file option in the file menu, through I personally find the generated directory listings easier to navigate than the open file dialog. Sadly, I'm not aware of any way to just insert the contents of a local file into a textbox without manually opening the file and copying its contents, and naturally, you'd need another GUI app for files that can't be opened directly in Firefox.
Hope that helps and sorry if some of this is super basic. I've been using firefox since before Firefox 1.0, longer than I've been using Linux and I was a long time Linux user when I went blind and I've never gotten the hang of using a text-mode web browser, so I really don't know how much of this is stuff you genuinely don't know coming from text browsers and how much is stuff I take for granted becuase I struggle to remember a time before I learned.