I have been using Tamil99 in LibreOffice, Firefox, Thunderbird, and gedit for some time now without difficulty. I thank everyone who posted responses to various queries. - Ramki (T. Ramakrishnan)
A) I received the following instructions from my friend, Karuna. These are an improvement over what I'd found earlier using help from those who posted replies to my query.
System Settings -> Language Support
For Keyboard Method Settings, Choose ibus and then
click Install/Remove Languages and choose Tamil.
From Dash Home, type Keyboard Input Methods and
you will get the iBus Preferences Window.
Click 'Show Input Method name on language bar' and
then under Input Method choose Tamil99.
Logout and login or to be safe, just restart the machine.
Now go to any application like gedit, LibreOffice, Thunderbird and type in English and then do Control+Space and then you will be able to type in Tamil. There will be a visual keyboard on the top menu bar when input language is English, and Tamil 'Tha' when typing in tamil.
I am not able to type many tamil characters in the shell; they show up as "boxed" (strange) characters.
The confusing part is that there is a System Settings -> Keyboard Layout which just has one entry now (English) and I don't need to do anything here (If I wanted to use some other keyboard like Tamil Unicode or Tamil TAM, then I could add that keyboard here, but since there is no support for Tamil99 here, we don't change anything here.)
You must not select "Use this method for all applications" in "Input Methods".
B) In order for the above procedure to work, we need to do the following first. (I'm not sure if we need all of the following libraries!)
Start Ubuntu Software Center.
Search for m17
Install m17n-db, m17n-contrib, ibus-m17n.
Install m17n-im-config, m17n-lib-bin.
Logout and log back in.
C) Everytime I boot the system or login, I start ibus.
Then press ctrl-space to switch between Tamil99 and English.
I couldn't make the daemon work so I am doing this everytime.
In other words, the following didn't work for me when I tried it a while ago:
Go to Dash Home.
Click "Startup Applications".
Command: /usr/bin/ibus-daemon -d
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