Single Key Tab Switch brings the 'Single-key shortcuts' feature of [Opera] 1 to [Mozilla Firefox] 2, letting you use (by default), 1 and 2 to go to previous and next tab, respectively.
The hotkeys can be configured by editing these properties in about:config:
extensions.skts.prevTabKey
extensions.skts.nextTabKey
By default, these are set to 1 and 2, respectively, but can be set to any
single character. The key corresponding to that character will then be the
key to press to go to the previous or next tab.
NOTE: Firefox needs to be restarted for changes to the properties to apply.
If you just want to try out the add-on, you should get it from [AMO] 3 (see Where?).
To package the extension for testing, run mkxpi.sh, or (if you lack a shell),
create a zip file containing the files in src/, and change the file extension
from .zip to .xpi. You can then drag the .xpi into the Add-ons window in
Firefox to install it.
Single Key Tab Switch can be downloaded from [AMO] 3. If you like (or don't like) the add-on, feel free to write a review for it on AMO.