According to the docs, Account.BillingStreet is updateable, so you might check the field level security for the running user to see if they're allowed to update that field.

Secondarily, a Visualforce Page and an Apex Controller, while more verbose then some javascript may be a better way to go, if only for the fact that APEX gets better support on this forum (I know, not much of a reason) and attention from Salesforce than the AJAX toolkit, which is a relic from the now deprecated S-Control family.

According to the docs, Account.BillingStreet is updateable, so you might check the field level security for the running user to see if they're allowed to update that field.

Secondarily, a Visualforce Page and an Apex Controller, while more verbose then some javascript may be a better way to go, if only for the fact that APEX gets better support on this forum (I know, not much of a reason) and attention from Salesforce than the AJAX toolkit, which is a relic from the now deprecated S-Control family. (Yes, not much of a reason, I know)