There are common words that I frequently mistype where my error is also a properly spelled word e.g. fro when I meant for. Since I will almost never use fro I like to have it flagged as a spelling error to reduce the number of spelling errors I miss when proofreading my work.
For LibreOffice and Mozilla Firefox it’s simply a matter of deleting the word from the built-in dictionary. One word of caution, be sure to use a good text editor for this because the files are very large and often use Unix line endings so Windows notepad will likely trash the file. Also because the files are in protected folders you will need to do the editing as administrator.
On my Windows 10 Pro PC the files are named en-US.dic and located in these directories.
C:\Program Files\LibreOffice 5\share\extensions\dict-en
C:\Program Files\Mozilla Firefox\dictionaries
Unfortunately the Chrome browser compiles that same file into the executable so you would need to compile the program yourself to edit the standard dictionary.
Fortunately OneNote for Windows 10, OneNote & Word on the web, and Teams all flag fro as misspelled. (Excel on the web doesn’t have spell checking)